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CONFIDENTIAL Serial N? 3(lt 


NAVY DEPARTMENT 
BUREAU OF YARDS AND DOCKS 
WASHINGTON, D. C. 

September 30, 1941 


Tests and Design of 

Bombproof Structures 

of Reinforced Concrete 


By 

Captain C. A. TREXEL 

Civil Engineer Corps 
United States Navy 



United States Government Printing Office • Washington • 1941 




: ■■ 




























Navy Department, 

Bureau of Yards and Docks, 
Washington, D. C., September SO, 1941. 
From: Chief of Bureau of Yards and Docks. 

To: All holders of this publication. 

Subject: Issue and use of Bureau of Yards and Docks Confidential 
Bulletin ‘‘Tests and Design of Bombproof Structures of Rein¬ 
forced Concrete.’’ 

Reference: (a) Navy Regulations. 

1. The subject publication has been classified as confidential 
because it contains matter which should not be generally divulged. 
It should be issued and used in accordance with the requirements of 
reference (a) except that, although confidential, it will be distributed 
by the Bureau of Yards and Docks. No quarterly reports will be 
required, but the Bureau may at any time require an accounting 
therefor. 

Ben Moreell, 

Rear Admiral (CEC), U. S. N. 


(Ill) 


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CONTENTS 


Page 


Introduction_ xi 

PART I- GENERAL_ 1 

Definition of Bombproof_ 1 

High Explosive Bombs_ 1 

Types and Sizes_ 1 

Striking Velocity_ 1 

Angle of Impact_ 3 

Sectional Pressure_ 4 

Penetration Formulae____ 4 

(а) Poncelet_ 4 

(б) Petry_ 5 

(c) Peres_ 5 

(d) Vieser_ 5 

(e) Revised Petry_ 6 

Explosion Effects_ 7 

Blast_ 7 

Splinter Effects_ 8 

Tamped Explosions_ 9 

Scabbing Effects_ 9 

PART II—DEVELOPMENT OF TEST PROCEDURE, EQUIP¬ 
MENT AND STRUCTURES_ 11 

Basic Bomb and Test Conditions Adopted_ 11 

Method of Testing- Impracticability of Bombing_ 11 

Small-scale Versus Full-size Tests_ 12 

Similitude—Practicability of Using Small-scale Models_ 12 

Selection of Scales_ 14 

Plan of Tests--- 15 

Equipment for Tests_ 16 

Guns and Projectiles_ 16 

Equipment to Measure Projectile Velocities_ 17 

Instruments for Tests-- 17 

Types and Locations_ 17 

M. 1. T. Gages_ 23 

Manganin Wire Gages_ 24 

Modified Hopkinson Bar Gages_ 24 

Displacement Gages_ 25 

Blast Gages_ 25 

Naval Proving Ground Facilities_ 27 

Test Structures_ 27 

Layout of Test Shelters- 27 

Design of Test Shelters- 30 

Types and Dimensions_ 30 

Details_ 30 

Arrangement of Reinforcing Steel- 30 

Materials_ 34 


Concrete Strengths_ 

Construction of Test Shelters 

Workmanship_ 

Difficulties_ 

Completion Dates- 

Additional Test Specimens___ 


(V) 




















































VI 


Page 

PART III—TEST RESULTS_ 40 

Shelter Tests _ 40 

Preliminary Test Slab_ 40 

Penetration Test_ 40 

Explosion Test_ 41 

A (a) Shelter_ 41 

Penetration Test_ 41 

Explosion Test_ 46 

A(b) Shelter_ 46 

Penetration Test_ 46 

Explosion Test_ 52 

A(c) Shelter_ 55 

Penetration Test_ 55 

Explosion Test_ 56 

A(d) Shelter_ 56 

Penetration Test_ 56 

Explosion Test_ 56 

B(a) Shelter_ 59 

Penetration Test_ 59 

Explosion Test_ 59 

B(b) Shelter_ 63 

Penetration Test_ 63 

Explosion Test_ 63 

B(c) Shelter_ 69 

Penetration Test_ 69 

Explosion Test_ 69 

B(d) Shelter_ 69 

Penetration Test_ 69 

Explosion Test_ 73 

Summary of Test Results on Shelters_ 73 

Penetration_ 73 

Explosion_ 73 

Summary of Instrument Results of Shelter Tests_ 74 

Ceiling Deflections_ 74 

Pilongation by Strain Gauges_ 75 

Blast Data_ 75 

Hopkinson Bar Data_ 76 

Supplementary Explosion Tests_ 76 

A(c) Shelter_ 76 

A(d) Shelter_ 76 

B(c) Shelter_ 79 

B(d) Shelter_ 79 

Summary of Supplementary Explosion Tests_ 79 

Additional Tests on Slabs _ 79 

Description of Slabs_ 82 

Test Conditions_ 82 

Penetration Tests_ 86 

C(aD Slab_ 86 

C(bD Slab_ 86 

C(cl) Slab_ 86 

C(dl) Slab_ 86 






















































VII 


PART III—TEST RESULTS—Continued. 

Additional Tests on Slabs^—C ontinued. Page 

Explosion Tests_ 86 

C(a2) Slab_ 86 

C(b2) Slab_ 86 

C(c2) Slab_ 91 

C(d2) Slab_ 91 

Summary of Additional Test Results on Slabs_ 96 

Penetration_ 96 

Explosion_ 96 

Further Penetration Test on Slabs_ 96 

Description of Slabs_ 96 

Penetration Tests_ 97 

D(al) Slab_ 97 

D(a2; Slab_ 97 

D(bl) Slab_ 97 

D(b2) Slab_ 97 

D(cl) Slab_ 101 

D(c2) Slab_ 101 

Summary of Further Penetration Tests on Slabs_ lOl 

Penetration and Explosion Test_ 101 

Tamped Explosion Tests on Shelters_ 106 

Positions and Amounts of Explosive Charges_ 106 

Tamped Explosion Tests_ 107 

Shelter B(c)_ 107 

Shelter A(c)_ 112 

Shelter B(d) _ __ 114 

Shelter B (a)_ 118 

Shelter A (a)_ 118 

Shelter A (b)_ 123 

Shelter B(b)_ 127 

Shelter A (d)_ 130 

Summary of Tamped Explosion Results- 133 

PART IV—DISCUSSION OF TEST RESULTS_ 134 

Impact Penetration Effects- 134 

Nature and Shape of Craters and Holes- 134 

Diameter of Craters and Holes- 134 

Scabbing_ 134 

Reinforcing_ 134 

Effect of Oblique Impact_ 134 

Adjustment of Penetration Values_ 137 

Striking Velocit}^- 137 

Obliquity_ 137 

Sectional Pressure_ 139 

Concrete Strengtli_ 139 

Penetration Coefficient for Reinforced Concrete- 140 

C'omparison with British Values- 140 

Effect of Slab Thickness on Penetration- 141 

Effect of Special Reinforcing- 145 

Effect of Antiscabbing Plates- 146 

Scale effect—Comparison of Models and Prototypes- 146 



















































VIII 


PART IV—DISCUSSION OF TEST RESULTS—Continued. Page 

Explosion Effects_ 147 

Nature of Craters_ 147 

Scabbing_ 148 

Reinforcing_ 148 

Effect of Slab Thickness on Explosion Penetration_ 148 

Calibration of Explosion Penetration Formula_ 148 

Comparison of Theoretical Limit Thicknesses with Test Results___ 149 

Effect of Special Reinforcing_ 150 

Effect of Antiscabbing Plates_ 150 

Scale Effect-—Comparison of Models and Prototypes_ 150 

Instrumental Results_ 151 

Concussion Effects_ 151 

Increased Air Pressure Inside Shelters_ 151 

Comparative Destruction by Demolition and Armor-piercing Bombs__ 152 

Tamped Explosion Effcts_ 152 

Over-all Damage_ 153 

Instrumental Results_ 153 

Displacement_ 154 

Acceleration_ 154 

Earth Pressures_ 157 

Comparison of Earth Pressures with Acceleration Values _ 157 

Scale Effect—Comparison of Models and Prototypes_ 158 

PART V—CONCLUSIONS AND RECOMMENDATIONS_ 159 

Impact Penetration Results_ 159 

Impact Penetration Coefficient_ 159 

Effect of Relative Slab Thickness_ 159 

Reinforcing_ 160 

Antiscabbing Plates_ 160 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures_ 160 

Explosion Results_ 160 

Penetration Coefficient and Limit Thicknesses_ 160 

Earth Fill Between Slabs_ 161 

Reinforcing and Antiscabbing Plates_ 161 

Effect on Concrete Edges_ 161 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures_ 161 

Instrumental Results_ 162 

Single Slab Roofs_ 162 

Increased Air Pressure Inside Shelters_ 162 

Instruments Recommended for Future Explosion Tests_ 162 

Over-all Results_ 163 

Double Slab Roofs_ 163 

Reinforcing_ 164 

Protection Against Demolition Bombs_ 164 

Protection Against Larger Bombs or Higher Striking Velocities 

than the Basic_ 164 

Antiscabbing Plates_ 164 

Shelter Entrances_ 164 

Tamped Explosion Results_ 165 

Over-all Damage_ 165 

Instrumental Results_ 165 

















































IX 


PART V—CONCLUSIONS AND RECOMMENDATIONS—Con. Page 

Tamped Explosion Results—C ontinued. 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures_ 165 

Protective Apron_ 166 

Protection of Eciuipment Against Tamped Explosion Effects_ 166 

Recommended Additional Tests_ 167 

Recommended Size of Slabs for Future Penetration and 

Explosion Tests_ 167 

PART VI—DESIGN OF A BOMBPROOF STRUCTURE_ 168 

Upper Roof Slabs_ 168 

Required Thickness of Ujiper Roof Slabs_ 168 

Increased Protection_ 169 

Vent Openings Between Roof Slabs_ 170 

Overhang of Upper Roof Slabs_ 170 

Reinforcing_•_ 170 

Lower Roof Slabs_ 172 

Required Thickness of Lower Roof Slabs__ _ 173 

Single Slab Roofs_ 173 

Required Thickness of Single Slabs_ 173 

Side Walls_ 174 

Required Thickness of Side Walls Above Ground_ 174 

Required Thickness of Side Walls Below Ground-- 175 

Protective Aprons_ 175 

Required Thickness of Protective Aprons_ 175 

Required Width of Protective Aprons_ 176 

Residual Velocity_ 176 

Floor Slabs__ 177 

Required Thickness of Floor Slabs- 177 

Supports and Mountings for Machinery and Equipment- 178 

Entrances to Bombproof Structures- 178 

Types of Bombproof Structures- 178 

Bombproof Power Plant- 178 

Bombproof Communication and Command Center- 182 

Bombproof Personnel Shelter_ 182 

Underground Bombproof Structures- 182 

Method of Constructing Bombproof Structures- 185 

Degree of Bombproof Protection Provided- 186 


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INTRODUCTION 


The program of tests of bombproof striictiires of reinforced concrete 
ilescribed in this bulletin was initiated by the Bureau of Yards and 
Docks in May 1940, because of the need for reliable data upon which 
to base the designs of certain bombproof structures which the Bureau 
then had in hand. Published information is available on penetration 
and explosion effects, most of it compiled in the British Home Office 
ARP Handbooks and in Civil Protection by Felix James Samuely 
and Conrad Wilson Hamann. Nearly all the basic theories and a 
considerable part of the data on penetration are, however, over 100 
years old, pertain inmost cases to materials other than reinforced con¬ 
crete, and are characterized by a lack of correlation of results of one 
investigator with those of another, which casts some doubt upon the 
reliability of the analytical interpretation when applied to reinforced 
concrete. Many of the data are presented without statement as to 
source and are unsupported by test results. 

Data were required on the most effective design and disposition of 
protective materials and the degree of protection required in rein¬ 
forced concrete construction to resist direct hits and resultant pene¬ 
tration and explosion effects of large-size high-explosive bombs; also 
on the effect of the concussion waves from the impact and explosion 
of bombs upon personnel and facilities within the bombproof struc¬ 
tures; and whether, as had been suggested, that the concussion waves 
in the concrete, when transmitted to the air inside the shelter, would 
result in such increase in air pressure as to be harmful to personnel; 
and finally on the degree of protection required to resist earth pres¬ 
sures from tamped explosions under floors and near underground 
walls of structures. 

For the purposes stated there was not required, nor would time have 
permitted, a program sufficiently comprehensive to verify or calibrate 
existing penetration formulae, although it was expected that some 
data in this regard would be developed. Nor was the effect of various 
strengths and densities of concrete and of various arrangements of 
reinforcing steel upon the resistance to penetration and scabbing 
explored, except to a limited extent to determine their general effect. 
These are matters for further tests, most probably at smaller calibers 
in laboratories, because of the number of specimens and tests and the 
great cost and time required if carried on at large calibers. 

Tests were set up to obtain the aforementioned basic data on four 
selected types of bombproof structures, at the Naval Proving Ground, 
Dahlgren, Va. 


(XI) 


XII 


Wliile previously acknowledged, the cooperation of the Bureau of 
Ordnance, Navy Department, and of the Naval Proving Ground, and 
their willingness to undertake the tests and make available the per¬ 
sonnel and facilities of the Naval Proving Ground at a time when the 
latter were heavily taxed with the naval aircraft and shipbuilding 
program, is here again acknowledged. Captain G. L. Schuyler, 
U. S. N., Bureau of Ordnance, Navy Department, made valuable 
suggestions for setting up the tests. Particular mention is also made 
of the contribution made to the research by Lt. Comdr. W. S. Parsons, 
U. S. N., experimental officer, and Dr. L. Thompson, head physicist, 
of the Naval Proving Ground and their associates. To their apprecia¬ 
tion of the problem, enthusiasm, energy, and helpfid suggestions 
during the experimental work, the successful and expeditious accom¬ 
plishment of the tests is in large part due. Dr. Thompson also devel¬ 
oped the instrumentation for the tests. 

Mr. A. Amirikian, designing engineer, Bureau of Yards and Docks, 
rendered valuable assistance in the development of the test program 
and structures, in the analysis and interpretation of test results, and 
in the preparation of designs for bombproof structures predicated on 
the findings and conclusions. 

September 30, 1941. C. A. T. 


PART I- GENERAL 

DEFINITION OF BOAIBPROOF 

“Bombproof” construction, as commonly used, has reference to 
protection against bombs of the high explosive type. It is a relative 
term which, to have meaning, must be accompanied by a description 
of the bomb which it is designed to withstand, its size, weight, type, 
striking velocity, obliquity of impact, and the explosive content. 
With this understanding, bombproof, as herein used, includes not only 
blast and splinter protection, but protection against direct hits of 
bombs as well. 

HIGH EXPI.OSIVE BOMBS 
Types and Sizes 

High explosive bombs are of several types and many sizes depending 
upon the use to which they are to be put: small (20, 50, and 100 
pounds) light or medium case fragmentation bombs with contact fuses 
for use against troops and planes on the ground; medium case demoli¬ 
tion or general purpose bombs (250, 500, 1,000, and 1,500 pounds) 
with either instantaneous or delayed action fuses for use against 
hangars, shops, power plants, bridges, submarines and other ships; 
heavy case armor-piercing or semi-armor-piercing bombs ( 1 , 000 , 
1,500, 2,000 pounds and up) for use against factories, fortifications, 
battleships, and special targets; and large (4,000 pounds and up) light 
case land mines with percussion fuses or magnetic detonation for use 
against concentrated groups of buildings. Land mines are usually 
dropped with parachutes so that the light case will not break on striking 
and result in a low-order detonation. 

The charge-weight ratio or percentage of explosive filler is in inverse 
ratio to the weight and thickness of the bomb case, being 50 to 60 
percent of the total bomb weight in the light case bombs and land 
mines, 25 to 35 percent in the medium case demolition or general 
purpose bombs, and 8)2 to 22 percent in the heavy case armor-piercing 
and semi-armor-piercing type of bombs. 

Striking Velocity 

The striking velocity of a bomb is dependent on the type of bomb and 
the height of fall.^ Except at low altitudes, it is for practical purposes, 
independent of the speed of the plane, and of the size of the bomb of a 
given type. If there were no air resistance and if the bomb were 

1 In the future, auxiliary propulsion may be used to supplement the acceleration due to gravity. 

( 1 ) 



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dropped vertically, the a])proxiinate velocity woidd b(‘ o-iven by the 
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V=^j■2gIJ=S^JH 

in which g = the acceleration due to gravity taken as .32 feet per 
second per second. 

H=ihe height of fall in feet. 

]/"=the striking velocity in feet per second. 

Theoretical velocities, neglecting air resistance, and aerodynamic 
characteristics, for heavy bombs dropped from various altitudes, are 
given in Fig. 1. The actual velocities are somewhat less, due to the 




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compressive and frictional resistances of the air, both ot which depend 
on the shape and weight of the bomb. They have tlie effect of limiting 
the velocity to a terminal or maximum possible value, no matter at 
what height the bomb is released, which varies with the type of bomb, 
being as a rule greater for heavy bombs. A 2-pound incendiary bomb 
might have a terminal velocity of oidy 400 feet per second, whereas a 
500-pound bomb might reach 1,000 feet per second, a 1,000-pound 
bomb 1,100 or 1,200 feet per second, and a 2,000-pound bomb 1,300 
feet or more per second. It is estimated that a 2,000-pound A. P. 
bomb released at 16,000 feet elevation would attain a striking velocity 
of approximately 1,000 feet per second. 


Angle of Impact 

Impact of bombs is almost always at an angle, whether released 
from high altitude level bombing, dive bombing, or hedge-hopping 
planes. The angle of incidence is dependent on the height and speed 
of the plane at the moment the bomb is released and also to some extent 
on the ballastic properties of the bomb. Fig. 1 gives the approxi¬ 
mate angle of impact of bombs released by level bombing planes flying 
at various altitudes and horizontal air speeds, air resistance, and the 
aerodynamic characteristics of the bombs being neglected. 

It is estimated that a 2,000-pound A. P. bomb, released from a plane 
flying horizontally 250 miles per hour at approximately 16,000 feet 
elevation, attains a striking velocity of approximately 1,000 feet per 
second, and strikes at an angle of impact of approximately 70°, or, 
therefore, with an obliquity of 20°. 


Table 1. —Data on homhs 


Type 

Weight 

Case 

Lengthi 

Diam¬ 

eter 

Sectional 

pressure 

Explosive 

Percent 

Pounds 


Pounds 


Inches 

Inches 

Lbs.Isq. ft. 



Test projectile A. P_ 

13 

H 


3.0 

265 

21.5 

2.8 

Test projectile AP. 

13. 75 

H 


3.0 

280 



Fragmentation. 

20 

M 

24.0 

5.0 

147 

20. 

4.0 

Test projectile A. P. - 

100. 75 

H 


6.0 

513 

22. 

22. 2 

German G. P_.... 

110 

L 

42.0 

8.0 

316 

50? 

55? 

Fragmentation. . . . 

119.5 

L 

39.5 

8.0 

342 

45.5 

54. 5 

German G. P_ .. 

220 

L 

51.0 

10.0 

403 

50? 

110? 

British G. P_ _ 

250 

M 

51.27 

9.7 

487 

26.4 

66 

British S. A. P_ 

250 

H 

50.0 

9.16 

546 

18.5 

46 

MKXIIG.P_ _ 

- 500 

L 

59.5 

14.0 

468 

50.4 

252 

British G. P_... _ 

500 

M 

68.7 

12.9 

551 

23.7 

119 

British S. A. P_ 

500 

H 

62.0 

11.5 

693 

19.0 

95 

German G.P_._ _ _ ... 

550 

L 

60.0 

14.0 

515 

50? 

275? 

MK IX Demolition... 

1,000 

L 

82. 77 

19.0 

508 

73 

731 

MK XIII G. P_ 

1,000 

L 

76.1 

17.7 

585 

53.7 

537 

British G. P._.__ 

1,000 

M 

85.5 

16.15 

703 

33.5 

335 

British S. A. P ... . _ 

1,000 

H 

82.0 

13. 75 

962 

16.0 

160 

German G.P_ . 

1,100 

L 

84.0 

18.0 

622 

50? 

550? 

Projectile 

1,400 

H 


14.0 

1,310 



British A. P__ .. 

1,500 

H 

86. 25 

14. 95 

1,231 

io.6 

159 

MK I A. P__ 

1,580 

H 

82.0 

14.0 

1,478 

16.0 

253 

British G. P__ 

2,000 

M 

100.5 

19.0 

1,016 

47.6 

952 

Prototype projectile . __ .. 

2,000 

H 


16.0 

1,432 

22.0 

440 

British A. P___ 

2,000 

H 

112.7 

13.45 

2,027 

8.4 

168 


With tail. 















































4 


It is evident that bombs may strike the sides of biiildiiig-s, and 
lliat pitched roofs may sustain normal bomb bits if the angle of roof 
slope is equal to the angle of incidence of the bomb. 

Sectional Pressure 

All penetration theories and formulae Aire based on the assumption 
that the depth of penetration of bombs or other projh'ctiles into a 
target, assuming the projectile case is able to withstand the impact 
and penetration without deforming, is directly proportional to the 
sectional pressure or ratio of weight to maximum cross-sectional area 
of the bomb, usually expressed in ])ounds per square foot or some¬ 
times in pounds per square inch. Sectional pressures for a number 
of types and sizes of bombs are given in Table 1. It will be noted 
that the sectional pressure is greater for heavy case than for lighter 
case bombs and also increases with the size of the bomb. Accord¬ 
ingly, a large size A. P. bomb, approximating the characteristics of a 
projectile, would be expected to produce maximum penetration. 

TEN E l K A I ION FOH M L LAE 


The following formulae for ])enetration were taken from the British 
ARP Handbook No. 5, and “Civil Protection,” the symbols being 
changed to secure uniformity of meaning: 

(a) Ponceht: log,(^l 

in which 11= weight of bomb in kgs. 

A = maximum cross section of bomb in square meters. 

.^7 = acceleration due to gravity in meters per second per 
second. 


> Tho Naval Proving Ground uses a penetration coeilicient for the limit velocity and limit thickness of 
armor, derived by dimensional consideration, in the form 




eVtd 


cos d 


Fi(el(i. 


d 

’ A/i/s cos d 


Field, e) 


in which e = plate thickness 

V'/,=Iimit velocity for complete penetration (perforation) 
rf = projectile diameter 
J\/=mass of projectile 

0=angle between tangent to trajectory of center of gravity of projectile and normal to plate 
/v=average energy per unit volume punched (di'fined by the physical properties of the armor). 
The contours giving probable value of F are drawn through points plotted to represent all available 
data for any given type of armor pentrated by a given type of projectile. In general, the contours may be 
expected to change somewhat as the design of projectile is varied, and different kinds of armor will of course 
have different F contours. If a projectile breaks or deforms during iieiudration, the indicated velocify 
limits may bi* very much greater than limits for undefornied projectiles and hence the F values would be 
much greater. 

Having F contours for a given projectile and type of armor it is then convenient to compute necessary 
thicknesses, for given conditions of impact, which will provide protection up to the given limit velocity for 
armor of average quality. The results api)ly for similar conditions (of eld and 6) at any scale wdthin the 
range of sizes of projectile used at the Proving Ground, provided the armor and projectiles are of similar 
types and deform similarly during penetration. 





5 


a and 6 = constants depending on the material penetrated. 

/" = a coefficient depending on the shape of the bomb (usu¬ 
ally taken as unity). 

T"= striking velocity in meters per second 
and *S'= depth of penetration in meters. 

ib) Feiry: 

in which k' = a constant depending on the material penetrated. 
f/i,=diameter of bomb, in centimeters. 

k' = a function of the velocity. (The formula for V' is not 
given in the Handbook.) 

F' = 10 X logic (1 + 50 X10-® F2). 

T" /S', and W are as in Poncelet’s formula. 

(c) Peres: S=^ 

A 

in which X = a coefficient depending on the shape of the bomb (usu¬ 
ally unity). 

lJL = a coefficient depending on the material penetrated. 

kinetic energy of bomb in meter kilograms. 

H = maximum cross sectional area of bomb in square centi¬ 
meters 

and S is as in Pocelet’s formula. 

(d) Vieser: S'=-*/— 

\ COT 

in which to = coefficient depending on the material penetrated. 

r = the cube compression strength of the material penetrated 
in thousands of kilograms per square meter. 

£*= kinetic energy in meter-thousand kilograms, 
and /S is as in Poncelet’s formula. 

No attempt has been made to review these formulae and accompany¬ 
ing experimental results, or to determine their comparative reliability 
and applicability. Prof. H. P. Robertson has done so,^ and states 
“_at the present time the writer is inclined definitely to lavor a work¬ 

ing hypothesis of the Poncelet type,’’ which takes into consideration 
the stress required to overcome both the cohesion of the material 
penetrated and the inertial resistance of the resulting detritus. 
The authors of “Civil Protection” state that “an examination of these 
fonnulae shows that the second (Petry’s, which is a modification of 
Poncelet’s formula,) is both the most consistent and handy in use.” 

' In “Terminal Ballistics,” National Research Council Committee on Passive Protection against Bomb¬ 
ing, interim report to the Chief of Engineers, United States Army. January 1941. 


420.">04°- 41-2 



6 


For ease of application, they have transcribed the constants into pound 
and incli units and have rearranged the formula into the following 
form: 

{e) Revised Retry: S=kPV" 

in which k— a constant depending on the material penetrated. 

(Values of k are given in Table 2, for the materials 
most likely to be involved). 

F*—the sectional pressure of the bomb. (Values are given 
in Table 1.7.2. of the Handbook for several cases; but 
they have been given here, in Table 1, in pound per 
square foot units in which they are to be used in the 
modified formula). 

V" = Si function of the velocity of impact. It is a factor 
without dimension and is given by the expression 

V" =logio( 1 + 215,000) 

Values for V" are set out in Table 3 for different 
values of the velocity of impact V (in feet per second), 
and are also shown graphically on the chart, Fig. 2, 
and aS= penetration, in feet. 

Table 2. — Values of k for use in Formula (e) 


Material 

Ft.3 lt).-i 

Limestone__ __ _ 

5. .38X10-3 

Concrete 1 .. _ _ 

7.99X10-3 

Reinforced concrete 2_ _ _ 

4. 76X10-3 

Specially-reinforced concrete 3__ 

2. 82X10-3 

Stone masonry . . . _ . . 

11. 72X10-3 

Brickwork. _ . .. . .. 

20. 48X10-3 

Sandy soil_ _ _ 

36.7 XlO-3 

Soil with vegetation_ .. ... _ 

48. 2 XlO-3 

Soft soil_ _ _ _ 

73.2 XlO-3 


1 Mass concrete with a crushing strength of 2,200 pounds per square inch. 

2 Normal reinforced concrete with a crushing strength of 3,200 pounds per square inch and 1.4 percent of 
reinforcement. 

3 Specially-reinforced concrete with a crushing strength of 5,700 pounds per square inch and 1.4 percent of 
reinforcement. 

Table 3. —Values of V" for use in formula (e) 


V 

(ft. per sec.) 

V" 

100 

0.019 

200 

.074 

300 

. 153 

400 

. 242 

500 

.335 

600 

.427 

700 

.515 

800 

.599 

900 

.678 

1,000 

.752 

1,100 

.825 

1,200 

.886 

1,300 

.947 




















7 



Blast 

Even less is known about explosion effects than about penetration. 
They are complex in nature and contingent upon the type of explosion: 
i. e., whether open (untamped) or confined (tamped). Open explosions 
result in blast and splinter effects. A blast wave is characterized by 
a higb-frequency compression front which, owing to its high fre¬ 
quency, is transmitted mainly in straight lines, followed by a low- 
frequency suction wave of greater duration which can pull out windows 
around the corner. The energy at the center of explosion is very 































































































































































































8 


high and even at a distance the blast wave may cause serious structural 
damage. An idea may be gained of the magnitude of these forces 
by referring to Table 4, obtained from British sources. 


Table A.—Blast 'pressures from bombs and TNT 


Explosive 

Distance 

Pressure 

1 

Suction 

Duration 

P. 

i 

550-pound German_ 

Feet 

50 

50 

50 

50 

50 

Lbs.hq. in. 
14.5 
17. 7 
0.0 
28.0 
29.0 

Lhs.isq. in. 
2.8 
2.9 
1.4 
3.0 
4.2 

Millisec. 

8.3 

9.8 

0.0 

8.0 

7.5 

1 

Millisec. 

41 

40 

30 

49 

45 

550-pound German_ .. 

500-pound British GP_ 

270-pound TNT_ 

270-pound TNT 



Note.— At 200 feet the pressure values are 7 percent of those at 50 feet, and the suction values 14 percent. 
The velocity of the blast wave decreases from 1,250 feet per second at 50 feet to 1,180 feet per second at 200 
feet. 


An explosion which occurs on top of a shelter roof is considered an 
open explosion. Such an explosion acts in much the same way as the 
kinetic energy of a falling bomb, but it affects a wider area. A crater 
is formed in the top of the slab and, if the energy of explosion is 
sufficient, scabbing will occur on the underside. “Civil Protection” 
gives the minimum depth which is just not perforated by an explosion 
as 

/S (in feet) = 2c' (in lbs.) 

in which C is the charge in pounds, and 

c' is a factor depending on materials, 
c' may be taken as 0.41 for ordinary ground; 

0.26 for mass concrete, with a crushing 
strength of 2,200 pounds per 
square inch; 

0.22 for normal reinforced concrete with 
a crushing strength of 3,200 
pounds per square inch and 1.4 
percent of reinforcement. 

0.19 for specially reinforced concrete 
with a crushing strength of 5,700 
pounds per square inch and 1.4 
percent of reinforcement. 

Splinter Effects 

Splinters or jagged fragments of steel resulting from the bursting of 
the bomb case are projected by the explosion at initial velocities as 
high as 8,000 feet per second and may cause death at 4,000 feet. The 
following thicknesses of materials are given in the ARP No. 5 Hand¬ 
book as being adequate to resist maximum splinter effects of a 
500-pound bomb bursting at 50 feet: 

















9 


Mild steel_ _ _ _ in. 

Stock bricks in cement__ 13}^ in. solid,' 

15}i in. cavity. 

Unreinforced concrete_ 15 in. 

Ordinarily-reinforced concrete_ 12 in. 

Specially-reinforced concrete__ 10 in. 

Sand or earth revetments__ _ _ _ __ 2 ft. 6 in. 


Experience indicates that, except in a few instances, these thick¬ 
nesses are ample. Walls of the thicknesses given may be expected to 
withstand blast pressures from the same bomb bursting 50 feet away. 

Tamped Explosions 

A tamped explosion, or one which occurs in a confined space that 
does not permit of free expansion of the gases, results in a crater if the 
gases of explosion reach the surface, or a ‘‘camouflet” where they do 
not. In either case an earth wave is set up having a destructive force 
that is reduced proportionally to the square of the distance from the 
center of the explosion. Formulae and discussion of this subject may 
be found in ‘‘Civil Protection.’’ In this connection, the following earth 
pressures taken from British reports, on underground structures due 
to explosions of 500-pound GP bombs containing 142 pounds TNT, 
will be of interest; 

(а) Pressure on 22K-inch walls of a brick basement in a chalk 
soil at 70, 50, 33, and 17 feet: 

Pressures were always greatest on the wall facing the bomb 
and next on the wall farthest from the bomb. At 50 feet or more, 
the pressure was slight; at 33 feet, nearly 10 pounds per square 
mch; at 17 feet about 30 pounds per square inch and the 22K-inch 
wall failed. 

Earth movement of the order of 1% inches to 4% inches 
occurred. 

(б) Pressure on walls of a reinforced concrete cell in sandy 
loam soil. 

The pressure at 45 feet was negligible; at 20 feet, 45 pounds 
per square inch; at 13K feet, 180 pounds per square inch. 

SCABBING EFFECTS 

When a projectile strikes a slab of material such as concrete, in 
addition to damage to the front face by penetration and explosion, 
if the energy be sufficient a portion of the rear face opposite the point 
of impact may be separated from the slab. This phenomenon is 
known as scabbing and in ARP Handbook No. 5 is ascribed to a 
reflected pressure wave. The authors of ‘'Civil Protection” suggest 
that the effect is due to tensile forces set up simultaneously with and 
at right angles to the impulse given to the material on the opposite 










10 


side of the slab. Professor Bernal in ^^London Engineering/^ De¬ 
cember 27, 1940, describes scabbing as being due to elastic compres¬ 
sion which causes a local disintegration at the surface opposite to 
the point of impact, projects the loose material onwards and thereby 
assists penetration by the resultant reduction in thickness. Whether 
scabbing will occur depends primarily on the nature of the material 
and whether sufficient energy is applied. Scabbing occurs in elastic, 
friable materials such as concrete or stone which have relatively low 
tensile strengths compared with their compressive strengths. While 
somewhat similar effects occur in some classes of armor plate, strictly 
speaking, scabbing does not occur in steel. The practical significance 
of scabbing in concrete is that a projectile at a given velocity is able 
to perforate a slab of a thickness considerably greater than the depth 
the same projectile and at the same velocity would be able to penetrate 
into a slab of great thickness. It is suggested in some of the ARP 
Handbooks that scabbing might be reduced by appropriate arrange¬ 
ment of reinforcing steel or by attaching a steel reflector plate to the 
under side of the slab. 


PART II—DEVELOPMENT OF TEST PROCE¬ 
DURE, EQUIPMENT AND STRUCTURES 

Basic Bomb and Test Conditions Adopted 

Since theoretically there is almost no limit to the size of bomb which 
may in the future be used, strictly speaking no structure which it is 
practical to build can be considered as absolutely bombproof. Ac¬ 
cordingly, in order to designate a given structure as ‘‘bombproof, 
it becomes necessary to select or define the size and type of bomb 
and striking velocity which it has been designed to withstand. For 
economic reasons, there is a limit to the degree of protection which it 
is practicable to provide and, therefore, to the maximum size of bomb 
against which protection can be provided. For the purposes of these 
tests and structures at present being designed, this maximum or basic 
bomb was assumed as a 2,000-pound heavy case, armor-piercing 
(A. P.) bomb. A. P. bombs in this size are not now in general use. 
An A. P. type of test bomb was desirable for a number of reasons: 
first, because it can be fired from a gun; second, a heavy case bomb 
was required to explore penetration efi’ects in concrete, because a 
lighter case bomb would have deformed or broken up; and, last, it 
was assumed that the A. P. type of bomb, even though containing 
much less explosive filler, would be more destructive against reinforced 
concrete structures than a lighter case bomb, because of its ability to 
penetrate prior to detonation. 

A terminal velocity of 1,000 feet per second, corresponding to release 
at 16,000 feet elevation, was selected as representative of average 
maximum service bombing conditions. An obliquity of impact for 
test purposes of 20° was also selected as being average. It corresponds 
to the angle at which a bomb would strike if released at 16,000 feet 
elevation from a plane flying horizontally at 250 miles per hour. 

Method of Testing—Impraeticability of Bombing 

Preliminary consideration of test procedure made it evident that 
carrying on the tests by actual bombing would be impractical. The 
pin point accuracy necessary to hit with any degree of regularity 
targets as small as the shelters, to say nothing of hitting them at the 
particular points of impact desired, from an altitude of 16,000 feet 
required for a striking velocity of 1,000 feet per second, is not attain¬ 
able. The wastage of A. P. bombs, diversion of bombing planes from 

( 11 ) 


12 


regular duties and of Naval Proving Ground personnel and facilities 
from other urgent work, and inability to obtain satisfactory measure¬ 
ments of striking velocities, to correlate the procedure of the bombing 
planc's with the personnel on the ground and generally to maintain 
satisfactory control of the experiments, also ruled out actual bombing 
as a practical test procedure. Discussion with Bureau of Ordnance 
and Naval Proving Ground personnel established the feasibility of 
firing heavy case A. P. bombs from guns. As such a method made it 
possible to utilize the regular instrumental control and experienced 
personnel of the Naval Proving Ground, it appeared to be a logical 
choice. In practice, it proved to be an economical, reliable and 
emmently satisfactory method. It involved one change from bomb¬ 
ing procedure; namely, rotation of the test structures from their 
normal position to one with the roofs fachig the guns. 

Small-scale Versus Full-size Tests 

While full-scale models are usually preferable for test purposes, 
reduced-scale models, assuming that conditions of similitude could be 
established, appeared to be a practical necessity for the tests con¬ 
templated. The cost and time required for full-size penetration and 
explosion tests, as con^prehensive as the reduced scale tests actually 
made, would have been prohibitive. Full-size tests would have re¬ 
quired an area too large to be pre-empted in the plate battery area at 
the Naval Proving Ground and would have resulted in interruptions 
to the work of the plate battery and some hazard to life and property 
in the vicinity. Time was also a major factor of importance which 
dictated that tests be completed and design determinations made as 
expeditiously as possible, because construction of bombproof struc¬ 
tures for which the test data were required had already begun. 

Similitude—Practicability of Using Small-scale 
Models 

The problem of obtaining conditions of similitude between reduced- 
scale models and prototypes appeared to be complicated by the fact 
that the same models were to be used to study both penetration and 
explosion effects, which follow different laws. Dimensional analysis 
indicated, however, that conditions of similitude were practicable of 
application and that penetration and explosion effects on the full-size 
prototype structures could, therefore, be reliably predicted from tests 
on reduced-scale models. From the basic penetration formulae we 
know that the variables, which sensibly influence the penetrative 
action, are the weight, maximum cross section, and shape of the 
projectile, its striking velocity and obliquity of impact, and tlu' charac¬ 
teristics of the target materials relating to resistance to penetration 
and scabbing, such as density, compressive, tensile and shear strengths 


13 


and modulus of elasticity. Other factors whose effects are minor 
or obscure, such as duration of impact, energy dissipated m heat, etc., 
may be ignored. It will be evident that the same permissible con¬ 
crete and steel stresses will apply to models and prototypes. It can 
be shown that, for equal stresses at corresponding points, the striking 
velocities (which include obliquity of impact) must be equal, and that 
the materials in models and prototypes must have the same elastic 
properties. It may be assumed that the reduction in scale of th(‘ 
models under consideration is not of such extent as to affect the elastic 
properties and other characteristics of the concrete materials; and, 
also, that the characteristics of the reinforcing steel in models and 
prototypes are likewise unchanged. The k and V" factors relating 
to nature of target materials and striking velocity of the projectile 
in the penetration formula S=kPV" may, accordingly, be elimi¬ 
nated and only the effect of the sectional pressure (P) upon the 
penetration given further consideration. If the subscript “p” be 
used to denote prototype measurements, “m” to denote model mea¬ 
surements, and “r” to denote scale ratios of model dimensions to 
prototype dimensions, then, from the penetration formula iS'=^PF". 


or, since P = 


weight 


Sr, Dr 


Sr = Dr. 


, - and the weight of an 

cross-sectional area 

AP projectile varies as the cube of the diameter, and 

the cross-sectional area varies as the square of the 

diameter. 


(in which P=projectile diameter) and 


This relation establishes that penetration varies directly as the pro¬ 
jectile diameter and, therefore, that if reduced-scale models are built 
to the same linear scale as the diameters of projectiles used, penetra¬ 
tion in the prototype structures can be predicted from the penetration 

in models, by applying the linear scale. _ 

Similarly, for explosion effects, from the explosion formula S=ac'%lC, 
(‘liminating the constants, 

yc; 

or, since the explosive charge is a fixed percentage of 
the weight of the projectile, which varies as the cube 
of the diameter, 

Sp, _ D m 

and again 

Sr = Dr. 



14 


This relation indicates, therefore, that the same models may be nsed 
to predict both penetration and explosion effects in the prototype 
structures, and that the scale ratio of models to prototypes should 
correspond to the ratio of diameters of the AP projectiles used against 
each. The following will, accordingly, establish conditions of simili¬ 
tude between models and prototypes: 

(а) Use the same striking velocities and oblicjuity of impact 
for models as are assumed for the prototy])es; 

(б) Use the same concrete and steel materials in the models 
as are proposed for the prototype structures; 

(c) Construct the models to the same linear scale as the ratio 
of the diameter of the projectile used against the models, to the 
diameter of the basic projectile for which the prototype struc¬ 
tures are to be designed. 

Selection of Scales 

Because of uncertainties unavoidably associated with reduced 
scales, two sets of scale models were considered necessary, in order to 
determine the scale effect, if any, and therefore to permit application 
of the quantitative results obtained with more certainty to the full-size 
prototypes. The scales adopted were dictated in part by time and 
cost considerations, but primarily by the sizes and weight ratios of 
available projectiles and availability of the corresponding guns to 
fire them. It was convenient to adopt 6-inch projectiles, because 
6-inch common projectiles similar to AP bombs were available, as 
was a suitable 6-inch gun. A 3-incli gun was also available, for which 
projectiles simulating AP bombs could readily be manufactured. 
The 3-inch projectiles weigh 13.0 pounds. Assuming the weight of 
the AP bombs, which are similar to projectiles, to vary as the cube of 
the diameter, a 2,000-pound AP bomb would be approximately 16 
inches in diameter. The actual diameter may be appreciably less 
than this if the explosive filler be reduced much below 20 percent, but 
was assumed to be near enough for the purposes of these tests. Since 
penetration varies as the sectional pressure, or, therefore, directly as 
the diameter of an AP type of bomb, then, assuming a basic 2,000- 
pound AP bomb to have a 16-inch diameter, the 6-inch AP bomb 
would correspond to a %-scale model, and the 3-inch projectile to a 
Ke-scale model, insofar as penetration effects are concerned. As 
explosion effects were also thought to vary approximately as the cube 
root of the weight of the explosive, it was expected that no appre¬ 
ciable scale effect would be encountered in the explosion tests. The 
%- and Ke-scale models were convenient from the standpoint of cost 
and time of construction and testing, and these' two scales were 
adopted for the tests. Within practicable limits, conditions of simili- 


15 


tilde were planned throughout in the dimensions of the models and 
in the percentage, size and spacing of the reinforcing steel, as 
described hereinafter. It was not considered necessary, with the size 
of projectiles used, to scale down the size of concrete aggregates, 
though this would have to be done for laboratory tests with small- 
caliber projectiles. 

Plan of Tests 

The plan of the tests was to construct in the plate battery area at 
the Naval Proving Ground, four types of, reinforced concrete test 
shelters, series A at % scale and series B at jU scale. The four types 
of roof construction selected—and the shelters were thus lettered for 
each scale—were: (a) a double slab with sand and earth fill between, 
with the toj) or outer slab of such thickness so that it would be per¬ 
forated and the effect of the explosion occurring between the two slabs 
might be determined; (b) the same as (a) but with a vented airspace 
substituted for the sand and earth fill; (c) a double roof slab with 
sand and earth fill between, but with the upper slab thickened to pre¬ 
vent perforation, and the lower slab made correspondingly thinner; 
and (d) a solid concrete slab. The combined total roof thickness was 
the same for the test shelters of one scale, only the distribution of the 
concrete in the slab or slabs varying. It was expected that the roof 
slabs in shelters (c) and (d) would not be perforated and that the 
tests on these shelters and on the preliminary test slab would furnish 
some data for calibrating penetration formulae for reinforced concrete. 
The shelters were to be rotated 70° from their normal position to en¬ 
able the roofs to be struck at an obliquity of approximately 20° with 
projectiles fired from guns. After a minimum ageing time of 30 days, 
the %-scale models were to be struck with inert 6-inch uncapped com¬ 
mon projectiles and the e-scale models with inert 3-inch uncapped 
A. P. projectiles, each at a point near the center of the shelter at a strik¬ 
ing velocity of 1,000 feet per second, intended to accomplish punching, 
without deformation of the projectiles. The penetration tests were 
to be followed by static detonation of charges of TNT simulating det¬ 
onation of a 22 percent filler, 5 caliber A. P. bomb in the position oc¬ 
cupied by each projectile at maximum penetration. It was assumed 
that maximum demolition of the structure would result when all the 
kinetic energy of the projectile had been expended and maximum 
penetration reached, before explosion occurred; accordingly, that 
penetration effects might be considered separately from explosion 
effects. The position chosen for detonating the charge was to be the 
most unfavorable to the shelter and to include considerations of axial 
position and support as well as depth of penetration. The amount of 
charge of TNT was to be based on a 22 percent filler, 5 caliber A. P. 


16 


bomb, 22.2 pounds for 6 inch diainet(‘i- and 2.8 pounds for 3 inch 
diameter. The ga^es and instruments for r(‘eording the displacement 
of, and sliock intensity and strain in tlie ceiliiig slab, and increased air 
pressure within the shelter due to the concussion wave, were to be 
designed and built by the Naval Proving Ground, and necessary ele¬ 
ments cast in the concrete as the test shelters were poured. Bm-eau 
of Standards step-by-step contactor type gages, recording by gal¬ 
vanometer oscillograph were proposed to determine the slab disi)lace- 
ment; modified Hopkinson bar gages to measure shock intensity; 
manganin wire and M. I. T. gages with cathode ray oscillograph to 
determine relative and actual strain in the several ty])es of roof con¬ 
struction ; and Williams, swinging door and pi(‘zo-(‘lectric gages re¬ 
cording by cathode oscillogiaph to determine the blast or incrc'ased 
air pressure inside the shelters. 

EQl Il'MENT FOK TESTS 
Guns and Projectiles 

The guns used were a 6-inch 47 caliber MK B No. 917 and a 3-inch 
23 caliber (boat) MK XIV No. 2426, shown on Fig. 3. With the 
low velocities adopted, it was unnecessary to provide permanent gun 



Fig. 3.— Guus used in tests. G-inch 47-calii)er Rim Mk li, No. 917 and 3-iuch 23-caliber (boat) Riin 

Mk XIV No. 242G. 






17 


foundations. The 6-inch gun was mounted with the trunnion axis 

10.4 leet above the ground with the base plate supported by a conning 
tower tube and anchored with six tie rods to armor grating under the 
tube. The 3-inch gun was mounted with the trunnion axis 4.7 feet 
above the ground. 

The 6-inch projectiles were common MK XXVII-4 uncapped, 

15.5 indies long, with the cavity filled with bird shot to bring the weight 
to 100.75 pounds. A hook was screwed in the base to provide for 
extraction. The 3-inch projectiles were Type A-1 A. P., uncapped, 
8.1 inches long, with the cavity empty, weight 13.0 pounds. A hook 
was provided in the base. 

For the explosions, TNT holders were made of ordinary steel pipe. 
They were filled with cast TNT. A %-inch diameter hole was bored 
down the center of the 2.8-pound charge, and a )^-inch diameter hoh* 
in the 22.2-pound charge, within 2 inches of the closed end, and this 
hole and the open end were filled with tetryl for a booster. The det¬ 
onation was initiated by a No. 8 blasting cap and the tetryl booster. 
The holders for the 2.8-pound charge were 3-inch diameter by 11 inches 
long, for the 22.2-pound charge 5-inch diameter by 21 inches long, and 
for the 8.4-pound and 66.6-pound charges used in the supplementary 
explosion tests, respectively 4-inch diameter by 13 inches long and 
8-inch diameter by 28 inches long. 

Equipment to Measure Projectile Velocities 

Projectile velocities were measured by standard velocity techuique 
using two solenoid coils and magnetic oscillograph recording. The 
shutters of this as well as other oscillographs used with the gages were 
opened by the breaking of a gun muzzle wire. It was expected that 
projectile velocities coidd be predicted to 25 feet per second, and 
measured under field conditions to an accuracy of 10 feet per second 
or, therefore, within a maximum range of error of ± 1 percent. 

INSTRUMENTS FOR TESTS ^ 

Types and locations 

The gages used in the shelter tests and their locations in the struc¬ 
tures are identified in Figs. 4 to 7, inclusive. The strain gages and 
the Hopkinson bar units are shown in Fig. 8. The AI. I. T. strain 
gages II, III were made up at the Proving Ground from A4. I. T. 
resistance elements, with the collaboration of Dr. A. C. Ruge of the 
Alassachusetts Institute of Technology, in a form suitable for im¬ 
bedding in the concrete. No. II was placed near the inner surface 
of the ceiling slab of each structure and No. Ill near the outer surface 


1 Description from Naval Proving Ground report. 



18 


4 
o-fo 


7"-III 12" 

n ®_® n 

2~3 

n.lll,2,a3 IMBEDDED 4" FROM INNER AND OUTER SURFACES 


5 DEEP 


5"DEEf 



SYMBOLS 

0—0 I 8 “MAN 6 ANIN WIRE GAUGE I LEFT, 2 NEAR INNER SURFACE, 3 NEAR OUTER SURFACE, 4 RIGHT 
0-0 9" MANGANIN WIRE GAUGE 1-2 - - - 3 - - .. ’ 4 - 

* MIT GAUGE I NEAR INNER SURFACE, TH NEAR OUTER SURFACE 

X SCRATCH GAUGE 

o deflection gauge 

• HOPKINSON BARS 4", 7", a 12" IN LENGTH I WEAK SPRING, II MEDIUM SPRING, 111 STRONG SPRING 

0 WILLIAMS GAUGE ALL FACING END WALL 

□ DOOR GAUGE 

O QUARTZ PIEZOELECTRIC GAUGE 

ALL STRAIN GAUGES WERE ORIENTED PARALLEL TO LONG AXES OF CEILINGS 

Fig. 4.—Types and locations of gages in Test Shelter A(a). 







19 




y— 



11 

V. 


II.TI.1,2,3 a 4 

WERE 

2" TO 2^ FROM 



SURFACES OF 

ROOF 



—O 



• 7-III 

>— 

1 



* • /fll 

4 




TI» • 12-11 





2 0 0 


\ _ 




y 



Fig. 5.—Types and locations of gag:es in Test Shelters A(b) and A(c). 




















20 



A(d) 


II WAS 5" FROM INNER SURFACE OF ROOF 


m " 

12" *' 

II 

II I. 

2 ■' 

1%** II II 

ti 

M tl 

3 '• 

18^ *• •• 

II 

M II 



GAUGE FACING ROOF 



B(a) 


2 WAS 3" FROM INNER SURFACE 

3,n am were 2A from surfaces 
OF roof 


quartz GAUGE 5? FROM END WALL FACING 
DOORWAY 

DOOR GAUGE So FROM END WALL FACING 
DOORWAY 



GAUGE FACING INNER ROOF 


Fig. 6.—Types and locations of gages in Test Shelters A(d) and B(a). 















21 



B(b) 


, DOOR GAUGE FACING INNE 
DOOR GAUGE 5^ FROM END WALL AND FACI 
QUARTZ GAUGE IN DOORWAY FACING OUTWA 
DOOR GAUGE IN DOORWAY FACING OUTWAR 


HID, 1,2,3, a 4 WERE 2 k FROM SURFACE 
OF ROOF. 





FROM INNER SURFACE OF ROOF 

H H M •• >• 

Ml II M It M 

U It II 11 II 


FACING ROOF 


Fig. 7. —Types and locations of gages in Test Shelters B(b), B(c), and B(d). 


420oU4°—41-3 























22 




Fig. 8.—Typos of instruments used in shelter tests. From top to bottom they are: 12-inch llopkinson bar, 
7-inch llopkinson bar, 4-inch Hopkinson bar with time piece attached, Manganin wire gage, and 
MIT gage. 


of the ceilino: slab, both being near the center of the ceiling area. 
The depth from the ceiling surface to the axis of the gage was 4 inches 
in the case of the %-scale structures and 2 }^ inches in the case of the 
Ke-scale structures. The inner ends of the Hopkinson bars were 
placed at depths approximately 2}^ inches, 5)2 inches and, in the case 
of structures A(a), A(b) and A(d), lOK inches. The long bar man¬ 
ganin wire strain gages were placed to show permanent strain at 
locations near the center of the ceiling surface and at positions ex¬ 
tending across the junctions of the ceiling and side walls. The center 
manganin gages were imbedded at the depths of the M. I. T. gages. 
Those extending over the side walls were placed at a depth of about 
5 inches from the inner ceiling surface for the %-scale structures and 
about 2 I 2 inches for the Ke-scale structures. Three types of blast 
gages were used. The Williams gage is the gage with which gun 
blast curves have been develo])ed. The swinging-door gage measures 
net impidse communicated to a small door mounted in a baffle. The 
piezo-electric gage provides a time record of pressure on the 4-inch 
diameter gage surface. An attempt was made to cement de Forest 
scratch gages (Baldwin-Southwark) directly to the inner ceiling sur¬ 
face after the structures were completed, but the results were unsatis¬ 
factory partly because of the difficulty in making a good contact 


23 


with the rough surface. It might be possible to prepare a special 
surface for the purpose, which would hold, but the motion of the ceiling 
involves such large accelerations and aggravated effects of shock that 
the usefulness of the gage in this work is doubtful. 


M. I. T. gages 

Details of assembly of the M. I. T. strain gages II and III are given 
by the sketch. Fig. 9. With two elements mounted symmetrically 
by cementing one on each side of the flat bar of the spool, only the 
effect of linear strain is recorded. The connection of the recording 
circuit is as shown in Fig. 9. Flexure of the bar produces in itself 
opposite and equal changes of resistance and hence opposite and 
equal changes of potential at the plates of the recording cathode tube. 
Records for these gages show time distribution of strain and because 
of the short duration of the test—a small fraction of a second—meas¬ 
urements of these records should give results free from important 
temperature effects. The records were obtained with a Dumont 175A 
cathode ray assembly, on a rotating drum film. The gages were 
calibrated by static loading to produce linear strain. Secondary cali¬ 
bration of the cathode recording system was made for each test. 


Spring Tim e Piece ^,,_ 

KAAA/\/\/lA/\AA/WWfff~^(y 1 I 

^Pointer 

HOPKINSON BAR GAUGE 


5" 

-T 


1 Bakelite Tube —; 

- 18 - 

Manganin Wire -7 



n 


¥ 



Brass 

Cap 


MANGANIN WIRE GAUGE 



Fio. 9.—Details of Uopkinson bar, Manganin wire and MIT gages. 































24 


Maiiganin wire gages 


Details of assembly of the manganin gages were given by the 
sketch, Fig. 9. The wire is attached to the two end pieces, which 
provide the positive coupling with the concrete at the distances de¬ 
fined by their separation as in the case of the spool for the AI. I. T. 
gage. This gage records by change of resistance, measured before 
and after the structure is exposed to the transient load, the combined 
effect of flexure and linear strain. The wire was placed under a 
slight initial tension so that it would register strain of either sign. 
The manganin bar gages imbedded in the %-scale structures were 
18 inches in length; those in the ^(e-scale structures were 9 inches in 
length. Mean strains over these distances in the concrete were 
obtained from the resistance coefficient of the wire. Measurements 
were taken immediately before and immediately after the transient 
loading to minimize the error from change of temperature. Measure¬ 
ments were made by means of a potentiometer circuit and d’Arsenval 
galvanometer set up in the field laboratory. 


Modified Hopkinson bar gages 


The Hopkinson bar gages were made with bars of three lengths, 
4 inches, 7 inches and 12 inches. The imbedded ends of these bars were 
placed at several depths in the concrete. The ‘Time pieces” were 1 
inch in length, consisting of a section of the steel of which the bar was 
made. The time pieces were held against the bars with springs of 
three orders of stiffness. It was intended that the range of spring 
constants would be sufficient to result in at least one gage having 
a reading intermediate in the scale range. The original Hopkinson 
bar was designed to measure mean pressure in the head of the shock 
wave by observing the velocity (momentum) communicated to the 
time piece. In the case of these experiments, the motion of the ceiling 
was found to be the predominating factor in communicating velocity 
to the time piece. There is no way to avoid this, since the rod was of 
necessity imbedded in the concrete. Increasing the sensitiveness of 
the spring merely increases the displacement produced by the ceiling 
velocity. However, these quantities are of some significance as relative 
measures of intensity of disturbance in the structures. The results 
obtained are indirectly a measure of shock intensity but were definite 
only in the comparison of structures A(c) and A(d). Presumably 
the same results were obtained for B(c) and B(d) but the scriber 
record was of poor quality for B(d). 


Displacement gages 


Tho timo records of deflection of the inner ceilings were obtained 
using Bureau of Standards step-by-step electrically recording dis¬ 
placement gages, recording by galvanometer oscillograph in the 
Proof Department oscillograph room. Each complete break-and-make 
sequence indicated O.Ol-inch displacement. These gages were attached 
to a heavy bar suspended by rope from wood pedestals resting on the 
bottom side of the shelter. Shock from the ceiling slab did not dis¬ 
place the bar until after the transient ceiling displacement of interest 
had taken place. These records give not only the displacements 
of the ceiling in certain tests but, being time-calibrated, permit obtain¬ 
ing velocity and acceleration data for the ceiling slabs. 

Blast gages 

The piezo-electric blast gage was recently constructed at the Proving 
Ground as a part of its program for development of a pressure-time 
gage specially suited for the measurement of gun-blast fields and the 
pressure waves associated with detonating explosives. The gage used 
in these tests has a diaphragm 4 inches in diameter, clamped at the 
periphery and supported at the center by a quartz crystal pile. 
Records were obtained with a General Radio cathode-ray oscillograph, 
recording on moving film, set up in the field laboratory. 

The Williams gage and the swinging-door gage measure quantities 
which should be fairly representative of the effective intensity of 
blast on service structures. Recent tests have shown that the Williams 
gage actually records pressures of the general order of those calculated 
as gage maxima for shock waves from detonating explosives. The 
gage has a light piston moving in a close-fitting tube against the 
normal air column. The swinging-door gage measures the net impulse 
of the positive and negative pressure increments communicated to 
the door. This would not be representative of the relative effect to 
be expected in the case of much lighter surfaces capable, because of 
small inertia, of responding more quickly to the blast loading than 
the door, or of sections of much greater effective inertia. The door 
gage can be designed to record small blast intensities, however, which 
cannot be recorded accurately by the other gages, and for similar 
conditions the relative deflections are useful measures of relative 
intensities. 

Fig. 10 shows the assembly of gages mounted near the center of 
the ceiling and the ‘‘inertia’’ supporting structure for the deflection 
gage. 


26 



Fig. 10.—Interior of Shelter A(c) showing the assembly of gages, and the “inertia” supporting structure for 

the deflection gage. 





27 


Time calibrations for the three cathode ray oscillographs were super¬ 
posed on the records b}/ introducing a coupling to one set of deflecting 
plates with a 1,000-cycle tuning fork circuit during a part of the 
interval of ‘‘open shutter/’ Cables were laid connecting all recording- 
devices to their respective recording systems, most of which extended 
from the shelter under test to the field laboratory in the large frag¬ 
mentation chamber. Cable and connections to the cathode ray 
oscillograph systems were screened to prevent electrostatic inter¬ 
ference, and were placed to minimize mechanical disturbance of the 
cable. Signals to the laboratories were arranged to permit control of 
fire from the field laboratory. The technique in obtaining records 
was that of usual oscillographic laboratory, employing moving film 
surface. Complete records were taken both for impact and for 
detonation. 

Naval Proving Ground Facilities 

In addition to the guns, projectiles, explosives and instruments 
described, and the plate battery area in which the bombproof shelters 
were constructed and tested, the Naval Proving Ground made avail¬ 
able all personnel and additional facilities required for the purposes of 
the tests. These included the experimental laboratory, shops, cranes 
to handle guns and test slabs, velocity coils, cables and recording 
instruments, fragmentation chambers and other facilities; also the 
personnel to mount the guns, rig firing circuits and velocity screens, 
install the gages in the test shelters, run circuits to and set up the 
recording instruments, manufacture the TNT cases, determine the 
explosive charges, make the calculations for gun elevations and firing 
positions, run all the tests, both impact and explosion, and photograph 
the results. Without these facilities, and the personnel experienced 
in their operation, it would have been impossible to complete the tests 
so expeditiously and successfully. 

TEST STRUCTURE 
Layout of Test Shelters 

The test shelters were laid out near the fragmentation chambers in 
the plate battery area, in the arrangement shown in Fig. 11. A 
bird’s-eye view and ground views of the layout are shown in Figs. 
12, 13, and 14. 


28 



Fig. 11.- Lay-out of test sholtors. 



Fig. 12.—Bird’s-eye view of test shelters. (The guns are at left center.) 







29 



Fig. 13.—Front view of test shelters. 



Fig. 14.—Side view of test shelters 









30 


Design of Test Shelters 
Types and dimensions 

As stated, two sets of test shelters, identified as Series A at % scale 
and Series B at scale, and four types of roof construction in each 
series were to be constructed. Referring to the dimensions of the 
full-size prototype and not to the dimensions of the scale models 
which were respectively % for the A series and jU for the B series of 
the dimensions of the prototype, the test shelters were based on a 
prototype structure 40 feet wide by 90 feet long by 39 feet high inside 
with 5-foot side walls, 4-foot bottom and the following roof construc¬ 
tion: (a) 4-foot upper slab and 5-foot lower slab with an 8-foot earth 
and sand fill between, (b) 4-foot upper slab and 5-foot lower slab with 
an 8-foot air space between and vent openings at the sides, (c) 7-foot 
upper slab and 2-foot lower slab with an 8-foot earth and sand fill 
between, and (d) a 9-foot solid slab. See Fig. 15. The structures 
were designed as rigid frame boxes with reinforcing continuous around 
corners. 

Details 

The shelters were to be rotated 70° from their normal position with 
the roofs toward the guns. The under side was half buried in the 
earth and the excavated material banked up behind the exposed part 
of the bottom. The roofs were, accordingly, well out of the ground 
and the sides largely unrestrained. On one side of each test shelter 
an access opening into the interior of the shelter was provided to per¬ 
mit inspection of instruments and of damage caused by the penetration 
of the projectiles and of the explosions. The ceilings inside were 
whitewashed to improve visibility and make cracks and scabbing 
damage more evident. 

Arrangement of reinforcing steel 


The reinforcing consisted of conventional two-way reinforcing top 
and bottom with usual bent stirrups and wired intersections. The 
amount of reinforcing steel in the prototype was determined from 
stresses due to static loading, consisting of dead load of the structure, 
earth-fill between roof and ceiling slabs, and the roof live load. The 
cross-section of the prototype was analyzed as a rigid frame, first 
considering the lower rectangular portion composed of the ceiling and 
floor slabs and the two walls as the initial bent supporting its own 
weight as well as the dead weight of the roof slab while shored, then 
the completed two-story bent under the forementioned conditions of 
loading. However, to assure a minimum desirable balance between 
concrete and steel, to the reinforcing thus obtained, sufficient bars 
were added to bring the percentage by volume of the main reinforcing 


31 



. 

MANHOLE, ONE END/Vja 

only-^^ ^ 


OPENINGS IN MODEL 
b) ONLY 


MAX.COMPUTED TOE PRESSURE 
2.5 TONS PER SQ. FT. 


Fig. 15.—Transverse sections and end elevation of ^^-scale test shelters (A series). 


























































































32 


-SYMMETRICAL ABOUT (t 

i" 

-LONG. REINF.-^ (J) (5) lO" O-C. 


BENT CORNER BARS (S) 
f @ 5" O.C. IN SAME 
PLANE AS WALL AND SLAB 
BARS. 



fNO.TWlRE GAGE (^D.) STIRRUPS 

9" O.C. (g) ,o» o.C .~ 

pLONG. REINF.^ ^ ® 10"O.C. 4-^ 

^TRANS. RElNF.§'(j) @ 5" O.C. "bj \ jr\ 

^-T-T —T 


TRANSVERSE SECTION 
SHELTER A(b) 


Fig. Ifi.—I'ransvorse section through typical Test Shelter A(b) showing reinforcing steel. 











































































































































33 


in the transverse and longitudinal directions up to 0.50 and 0.25, 
respectively. Due to the limited number of commercially available 
sizes of bars and the necessity of maintaining a minimum spacing for 
the concrete pour, the percentage of the reinforcing in the models 
varied as shown in Table 5. 


Table 5. — Percentage by volume of total reinforcing in roof slabs 


Location 

Direction of reinforcing 

Series A 

Series B 

a and b 

c 

d 

a and b 

c 

d 


fTransferse,_ 

0.68 

0. 59 

0. 61 

0. 61 

0. 64 

0. 62 


1 Longitudinal _ 

.22 

.20 

. 22 

.30 

.32 

.30 

Inner slab... . _ 

/Transverse... 

.80 

.88 


.90 

1. 20 


/Longitudinal.. _ . 

.40 

.24 


. 44 

. 28 



fVertical. ___ . _ 

.80 

.80 

.80 

.90 

.90 

.90 

'» ^ilS___ 

/Horizontal_ .. _ 

..34 

.34 

.34 

.33 

.33 

.33 


fTransverse ..._ . 

.69 

.69 

.69 

.61 

.61 

.61 

r lOOr SlB^D _ __ . 

/Longitudinal.. . __ 

. 22 

.22 

. 22 

.30 

.30 

.30 


The arrangement of the reinforcing is shown in Fig. 16 and the 
sizes and spacing of the main bars are listed in Table 6. 

Table 6. —Size and spacing of main reinforcing in each face of slab 


Location 

Direction of 


Series A 



Series B 


reinforcing 

a and b 

c 

d 

a and b 

c 

d 

Outer slab__ 

/Transverse.. . .. 


5^@5" >_ 

Vs(&5" 2. 

ym"— 

yim"— 

H®4". 

/Longitudinal_ 

yimo"— 

ymio" _ 

^@10"._ 



H@8". 

Inner slab_ . 

(Transverse ... 

[Longitudinal-. . 
/Vertical 

^@10"... 

Vsm"—- 

top. 
54@5"—bot. 

H@\0" _ 

H@5" _ 


M@4"— 

M@4"— 

Vom”— 

H@4"— 

H@4". 

V ails -- -- 

/Horizontal__ 

Hm” _ 


»/^@6"... 


Floor slab_ 

/Transverse_ 


5^@5"_ 


Hm”— 

H@4"— 

H@4". 

/Longitudinal 


H@10" 

1/2® 10". 

^@8"— 

^^@8" 

K@8". 


' An extra layer at mid depth of slab. 

2 Two extra layers located at one-half and two-thirds of depth. 


It is to be.noted that while the main reinforcing of the prototype 
was placed in two layers in each face, the corresponding steel in the 
models was placed in one layer. In addition, due to the practical 
consideration of minimum spacing, the relative bond value of the 
reinforcing in the models is somewhat lower. 

Only a nominal amount of web reinforcing was used in the proto¬ 
type, consisting of / 2 -inch round stirrups spaced 2 feet and 3 feet on- 
centers in the slabs and walls, respectively. In designing the corre¬ 
sponding steel for the models, the diameters and the spacings were 
reduced in direct ratio to the model scale. However, during the 
fabrication of the models, due to the nonavailability of small gage 
wires in stock, K-inch round stirrups were substituted for the No. 7 
and No. 13 gage wires called for by the design, and shown in Fig. 16. 

































































34 


Materials 

The reinforcing steel was an intermediate grade O. H. steel from 
the Cambria plant of the Bethlehem Steel Co., having a yield point 
of 44,000 to 52,000, a tensile strength of 71,000 to 82,000 pounds per 
square inch, and an elongation of 19 to 26 percent. The sand and 
gravel were furnished by the Massaponax Sand & Gravel Corporation, 
Richmond, Va. The gravel was of 1-inch maximum size having a 
specific gravity of 2.66, and weights dry and rodded of 107.6, and 
dry and loose of 100.3 pounds per cubic foot. The characteristics 
of the concrete sand which was 94.09 percent silica (Si02) and 0.15 
percent calcium (as CaO) are given in Table 7. 


Table 7. — Concrete and test report 


Test data Percent 

Clay lumps_ 0. 00 

Coal and lignite_ 0. 00 

Elutriation (clay and silt)_ 1. 15 

Organic matter (plate)_ O.K. 

Specific gravity_ 2. 64 

Weight per cubic foot: Pounds 

Dry and rodded_ 102. 00 

Dry and loose_ 94. 75 


Sieve analysis Percent 

Passing 100 mesh_ 3. 35 

Passing 50 mesh_ 11. 50 

Passing 30 mesh_ 40. 75 

Passing 20 mesh_ 57. 90 

Passing 16 mesh_ 72. 10 

Passing 8 mesh_ 92. 90 

Passing 4 mesh_ 100. 00 

Fineness modulus_ 2. 79 


The cement was “INCOR,” a high early strength cement of the 
Lone Star Cement Corporation. 


Concrete strengths 

The concrete selected for the test shelters was a class E, of a nominal 
compressive strength of 3,000 pounds per square inch. The strengths, 
as evidenced by test cylinders, together with the proportions and 
other data on the concrete are given in Table 8. It is estimated that 
the 28-day strengths were in the range from 4,500 to 5,000 pounds 
per square inch. 


















Tests of concrete cylinders 


35 


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36 


Construction of Test Shelters 

Workmanship 

Shelters were constructed in the locations shown in Fig. 11. The 
bottom slab—in reality the side wall—was poured first, with the wall 
steel projecting upwardly. Dowels were placed in the bottom slab 
for attachment of the side walls, and angle corner bars were likewise 
placed in the slab to turn into the front faces to be tested and the 
back wall. Angle bars were also placed in the four vertical corners. 
Views of the reinforcing steel and of the shelters in vaiious stages of 
construction are shown in Fig. 17, 18, 19 and 20. In two of the 
shelters with earth and sand fills, A(c) and B(a), the fill was placed 
through a hole left for the purpose and subsequently closed. In 
shelters A (a) and B(c), pouring of the top slab was deferred until 
the fill had been placed. In all other shelters, the walls and top 
slabs were poured monolithically. The Hopkinson bar gages and 
strain gages were placed in the concrete as pouring progressed. All 
concrete was spaded and vibrated as it was being placed, and was 
cured for 3 days by wetting down and covering with burlap. 

Difficulties 

A few construction difficulties were encountered. Erection of 
formwork and placing of reinforcing steel was troublesome, because 
the structures were built on an angle, and placing of concrete in the 
small shelters was difficult due to this angularity, the thin wall sec¬ 
tions, and the large amount of reinforcing steel. Periodic inter¬ 
ruptions due to firing of the plate battery added to the difficulties 
of placing concrete, and to costs as well. Some honeycombing 
resulted, but this condition was not serious. 

Completion dates 

A summary of the completion dates of the shelters, together with 
the thicknesses of the roof slabs which were tested, is given in Table 9. 


T.^ble 9 



Date com¬ 

Roof thickness 

Space between roofs 

Shelter 

pleted 

Outer 

Inner 

A(a)_ 

A(b) - - - 

Oct. 4.1940 
Sept. 20, 1940 
Oct. 11,1940 
Oct. 18,1940 
Oct. 12,1940 
Sept. 25,1940 
Oct. 25,1940 
Oct. 25,1940 

Inches 

18 

18 

31H 

40>^ 

9 

9 

15^ 

20H 

Inches 

1 22^ 

1 22}4 

9 

36 inches (earth). 

36 inches (air). 

36 inches (earth). 

18 inches (earth). 

18 inches (air). 

18 inches (earth). 

A(c) ^ _ - 

A(d)_ - . 

B(a)- 

B(b)_ 

B(c) ^ _ 

11^ 

4H 

B(d)_ 





















37 



Fir,. 17.—Roinforcing steel in top slab of Test Shelter Afd). 



Fir,. 18.—Test Shelters A(a) and A(b) under construction. 
-42()r)04°- -41-4 









38 



Fig. 19.— Test Shelter A(c) under construction. 



Fig. 20.—Reinforcing steel in place in Test Shelter B(a). 










39 


Additional Test Specimens 

During the construction of the shelters, six samples of the concrete 
were obtained for curing and shipment to Prof. Walker Bleakney, 
Palmer Physical Laboratory, Princeton University, for small caliber 
penetration tests being conducted for the National Defense Research 
Committee. To obtain some information in advance of the tests 
on the shelters on the general behavior of reinforced concrete slabs in 
resisting penetration and explosion effects, and to determine whether 
3-inch common projectiles would deform on impact with concrete, 
a slab 60 by 60 by 16K inches thick was poured on October 18, 1940, 
using the same concrete materials which were used in the shelters. 
The reinforcing was similar to that in the top slab of test shelter A(b). 
In addition, nine small test slabs were made of reinforced concrete, 
6 by 6 inches to 8 by 8 inches in cross section and 14 to 24 inches in 
length for the purpose of checking performance of the bar (shock) 
gages and the strain gages. One of these gages is similar to the 
“Hopkinson Bar” except that the time piece, whose momentum is a 
measure of shock intensity, is displaced against a spring in a receiving 
tube. By detonating small charges of TNT {% to pound) at various 
small distances from the surfaces of the test slabs, it was possible 
to select springs estimated to be suitable for recording intensities 
of shock from detonations of charges at expected positions above 
the ceiling slabs. One gage in each shelter was given a spring more 
sensitive than the other two, to show relative intensities of shock 
from the projectile impacts with ceiling slabs. When the test charges 
were detonated in direct contact with these small slabs, a considerably 
greater portion of the slab was demolished. 


PART III TEST RESULTS 


SHELTER TESTS 

Preliminary Test Slab, Poured October 18, 1940 

Penetration test, October 31, 1940, at 902 feet per second striking 
velocity and 18° obliquity with a 3-inch 13.0-poiind projectile. The pro¬ 
jectile penetrated the slab 6K inches, rebounded, and came to rest 1 
foot in front of the slab. The crater, which was roughly circular and 
dished in contour, varied in width from 17 to 19 inches. The rein¬ 
forcing bars were bent outward and one was sheared off, as were sev¬ 
eral stirrups. A through crack 47 inches long extended from below 
the crater to the top of the slab, but no scabbing occurred on the rear 
face. A view of the front of the slab, a close-up view of the penetra¬ 
tion crater, and a view of the rear face showing the crack, are given in 
Figs. 21, 22, and 23. 



Fig. 21.—Impact penetration test on Preliminary Test Slab. Projectile 3-inch 13 pounds; striking velocity 
902 feet per second; obliquity 18°; penetration 6H inches. 

(40) 



41 





Fig. 22.—renetration crater in Freliniinary 'I'e.'^t Slab. 

Explosion test, November 5, 1940, with 2.8 pounds TNT and 0.1 
pound tetryl (booster) detonated H. O. in projectile impact crater. 
The TNT holder was secured in the crater behind the reinforcing bars 
exposed by the penetration test as shown in Fig. 24. The earth sup¬ 
port was removed from the rear at the center over an area 2 feet in 
diameter. The noise of the explosion was not as loud as was expected, 
but the black smoke and appearance of the crater indicated high order 
(H. O.) detonation. The crater was deepened from 6^ to 9 inches 
and enlarged somewhat to 22 inches wide by 28 inches high. All the 
exposed reinforcing bars wei^e whipped around and back against the 
slab, many radial cracks were formed, one corner was broken off, and 
the scabbing on the rear face was severe. Views of the front and rear 
faces of the slab are given in Figs. 25 and 26. 

A(a) Shelter, Poured Oetober 4, 1940 

Penetration test, November 5, 1940, at 976 feet per second striking 
velocity and 20° obliquity, with a 6-inch 100.75-pound projectile. 
The 6-inch projectile perforated the 18-inch outer roof, passed through 
the earth fill, struck the inner roof slab, and lodged in the fill. It 
could not be located by probing. A motion-picture record of the 
projectile in flight and of the impact is given in Fig. 27. A view of 
the roof and a close-up view of the crater after impact are given in 



42 



Fig. 23.—View of rear face of I’rcliininary Test Slab showing 47-inch through crack, following impact pene¬ 
tration test. 



Fig. 24.— View showing method of securing holder containing 2.8 pounds TNT preparatory to detonation, 







43 



Fig. 25.—Front view of Preliminary 'I’est Slab following detonation of 2.8 pounds TNT in 6H-inch pene¬ 
tration crater. 



Fig. 26.—View of rear face of Preliminary Test Slab showing scabbing due to detonation of 2.8 pounds TNT 

in penetrat ion crater on front face. 



44 



Fig. 27.—Motion-picture record of impact penetration tests on Shelters B(b) and A(a) taken by Mitchel camera 100 frames per second. 
Left-hand six vertical strips show the 3-inch projectile enterins the field, striking the sh('ll('r and escaping at the top. The right-hand two 
strips siiow the impact of the B-inch projertile on Shelter A (a). 



































Fig. 29.- Close-up view of impact penetration crater in Shelter A(a). 
















46 


Figs. 28 and 29. The hole was similar to the one made in the A(b) 
shelter, which was shot first, the top of the opening being about lOK 
inches from the outside face of the slab and the bottom 7K inches. 
Small concrete fragments were projected back from the impact crater 
half way to the gun. The concrete in this slab appeared to be of only 
fair quality. It had the appearance of a wet over-sanded mix result¬ 
ing in a poor paste and some voids. There were no visible aggregate 
fractures, indicating that the bond between paste and gravel may have 
been poor. On the ceiling inside the shelter, a small area about IK 
inches in diameter of the cement surface and whitewash coating had 
flaked off opposite the point of impact and there were a few hair cracks 
radiating from this point; also one vertical crack about 8 feet long, 2 
feet to the right of the point of impact. A contour sketch of the crater 
formed in the outer slab is given in Fig. 30. No readings were obtained 
with the blast and bar gages, except the bar gage with light spring which 
recorded a deflection of 0.1 inch. The ceiling displacement gage 
recorded a displacement of 0.03 inch about 0.012 second after impact 
with the outer slab, as in the case of the impact on the A(b) shelter 
on October 31, 1940. 

Explosion test, November 27, 1940, with 22.2 pounds TNT and 
0.2 pound tetryl detonated H. O. in the crater in the muddy earth 
between slabs and in contact with the top of inner slab. The explo¬ 
sion did greater damage by far to this shelter than to any other. A 
large camouflet was formed in the mud between the slabs, the entire 
outer roof slab was badly cracked and bulged outward, and severe 
scabbing occurred on the ceiling inside the shelter, as may be noted in 
Figs. 31, 32, and 33. The TNT charge had been placed in a hole in 
the muddy earth between the slabs in contact with the inner slab. 
Since severe damage or destruction of the inner slab was anticipated, 
no instruments were set up inside this shelter. The resistance gages 
were broken. A Williams gage in the doorway gave a reading of 
% pound and a quartz piezo gage at the same location showed a reading 
of 1.9 pounds per square inch, which was appreciable but not serious. 

A(b) Shelter, Poured September 20, 1940 

Penetration test, October 31, 1940, at 998 feet per second striking 
velocity and 22° obliquity with a 6-inch 100.75-pound projectile. 
The 6-inch projectile perforated the 18-inch outer roof, was deflected 
upward, struck the inner roof (penetration 2% inch) with a ricochet 
impact, and escaped through the middle 2 feet by 4 feet opening 
between roofs. The projectile was not found and was assumed to have 
fallen in the river. A view of the roof after impact, a close-up view 
of the crater, and a view between the roof slabs are given in Figs. 
34, 35, and 36. The hole, where the projectile perforated the outer 


47 


r^gure5 reodod/e from fhio d/de 
indicate depth of scabbing m inches 




36r- 



I' 

^ 'i 


s 




^ ^ 

^ ''I 

"§ ^ 

N' 

^ 5f> 

^ ^ 5? 

*0 


6 [Z !g 24 30 36 

Fig. 30.—Contour sketch of impact penetration crater in Shelter A(a). 



Fig. 31 .—View of bulge in outer roof slab of Shelter A (a) following detonation of 22.2 pounds TNT in the 

muddy earth between roof slabs. 















48 



Fig. 32.—View of outer roof slab of SlK'lter A (a), after cietonation of 22.2 pounds TXT between roof slaf)s. 



Fig. 33.—Ceiling of Shelter A(a) showing the .severe scabbing from detonation of 22.2 pounds TNT in contact 

with the opposite face of the ceiling slab. 







49 



Fig. 34.—Impact penetration test on Shelter A(b). I'rojeetile 6-ineh 100.7o pounds; striking velocity 998 
feet per second; obliquity 22°; penetration outer slab complete, inner slab 2 % inches. 



Fig. 35.—Close-up view of impact penetration crater in Shelter A(b) 






50 



jPiG. 36.—View between roof slabs of Shelter A(b) after the impact penetration test. The projectile was 
deflected by the inner slab and escaped through an opening at the top. 








51 


Figures readah/e from this side 
indicate depfd of xabb/ng m inches. 



% I 






4 ) 


I 

‘O 

I 


'1:? 

r> ^ 
^ I 
-I 

^ § 

^ g 

I 

;u 

<=1- ^ 


Fig. 37.—Contour sketch of the impact penetration crater, outer face of the outer slab in Shelter A(b). 


slab, was elliptical in shape with the long axis vertical, the plane of the 
hole tilted inward at the top, with the top of the hole about 9% inches 
from the outside face of the slab and the bottom about inches, 
the scabbing at the rear being more pronounced. The reinforcing 
bars were bowed out with respect to the outer and inner faces of the 
slab but none were sheared off. On the ceiling inside the shelter, one 
vertical crack 2 feet long appeared 2 feet to the right of the point 
of impact. Contour sketches of the craters formed in the outer 
and inner roof slabs are given in Figs. 37, 38, and 39. None of the 
blast, shock, or strain gages indicated measurable deflection, though 
the ceiling displacement gage piston moved inward by a small amount 
(about 0.03 inch) in 0.002 second and back to zero position in 0.005 
second. The beginning of ceiling motion occurred about 0.012 second 
after impact of the projectile on the outer slab. Two Williams blast 
gages and two swinging-door blast gages were placed in the chamber 






52 


close to the center of the ceiling during this test and during the tests 
of November 5. These gages would show indication of pressures less 
than 1 pound per square inch. 

Explosion test, November 27, 1940, with 22.2 pounds of TNT 
and 0.2 pound tetryl detonated H.O. in the projectile impact crater 
on the inner slab. The explosion had practically no effect on the outer 
slab but the crater on the upper face of the inner slab in which the 
TNT holder was placed, was enlarged from 19 by 22 inches to 



I 

I 


^ I 

^ b k 


I I 1 

H I 


Figured reodod/e from thi5 3/de 
indicate depth at xabbing m inches ' .. 

Fig. 38.—Contour sketch of the impact penetration crater, rear face of the outer slab in Shelter A(b). 






53 




0 


igured reodod/e from thro 3/de 
: nd/cofe depff? of xabd/ng m inches , 

3 6 9 12 15 18 


21 



I 

50 


I 

u 


'b 

^ ^ I: 




11 ^ 
^ 50 


I 

% 


I 


!0 

I 


Qj 


Pit;. 39—Contour sketch of the impact penetration crater, outer face of the inner slab in Shelter A(b). 



Ki. 40.—Ceiling of Shelter A(b). showing the severe scabbing from detonation ol 22.2 pounds TN1 m the 
penetration crater on the outer face of the ceiling slab. 


420504°—41— 


o 

















54 



Fig. 41.—Impact pi'iK'tratioii tost on Shelter A(c). Projectile h-inch 100.75 pounds; striking velocity 
1,023 feet per second; obliquity 21°30', penetration 44'/'^ inches. 



Figures reod(:F)/e from ffus Sfo/e 
tndcafe depffi xabbmq m mebeb 



30|-J— 


46 


1 

11 
11 

Ml 

It ^ 

S •§ 




Vj 




Fig. 42. —Contour sketch of the impact penetration crater in Shelter A(c). 
















55 



Fig. 43.— View of the outer roof slab of Shelter A(c) after detonation of 22.2 pounds 'rX4' in the impact 

penetration crater. 

37 by 51 inches and deepened from 2% inches to 20K inches, or nearly 
through the 22K-inch slab. A large area of the ceiling scabbed off, 
exposing the reinforcing steel. As in the shelters A(a) and B(b), the 
scabbed area was longer in the direction of the transverse reinforcing 
bars which were nearest the inside surface. See Fig. 40. On detona¬ 
tion, the scaffolding holding the blast gages was knocked backward by 
flying concrete, and no reading of the deflection gage was obtained. 
The filaments of both light bulbs were broken, probably by flying 
concrete. One light bulb was broken, but the glass was not damaged 
on the other. The piezo-electric gage facing the open doorway 
registered the large pressure reading of 8 pounds per square inch, 
undoubtedly from the blast through the door opening. The M. I. T. 
resistance gage II showed a tension strain of 0.072 percent, followed by 
a compression strain of 0.045 percent. Gage III showed a compression 
strain of 0.045 percent. The manganin resistance gages were broken. 

A(c) Shelter, Poured October 11, 1940 

Penetration test, November 27, 1940, at 1,023 feet per second 
striking velocity and 21° 30' obliquity, with a 6-inch 100.75-pound 
projectile. The 6-inch projectile penetrated the 31 K-inch outer roof slab 
14K inches, rebounded, and fell in front of the shelter. A close-up view 
of the penetration crater and also of the 6-inch projectile is given in 
Fig. 41, and a contour sketch of the crater is shown in Fig. 42. There 





56 


were no visible cracks or scabbing on the ceiling inside the shelter. 
The deflection gage showed a maximum reading of 0.01 inch, reached 
in 0.013 second after impact. The manganin gage 2 showed a compres¬ 
sion strain of 0.05 percent and M. I. T. gage II, 0.072 percent compres¬ 
sion strain. Williams gage 7 showed % pound and the swinging-door 
gage 3 registered some indication of small effect. 

Explosion test, November 27, 1940, with 22.2 pounds TNT and 0.2 
pound tetryl detonated H. O. in the projectile impact crater. The 
explosion enlarged the crater from 34 by 58 inches to 65 by 74 inches 
and deepened it from 14K inches all the way to the reinforcing steel on 
the rear face, or completely through for all practical purposes. There 
were also radial cracks from the crater. The reinforcing bars were 
broken and flattened back against the slab. See Fig. 43. There 
was undoubtedly scabbing of the inner face of the outer slab though 
this face was, of course, not available for inspection. One circum¬ 
ferential crack 3 feet long, 4 feet to the right and below the axis of the 
explosion appeared on the ceiling inside the shelter. The deflection 
gage registered a ceiling deflection of 0.20 inch, reached 0.013 second 
after impact. The ceiling slab had an initial velocity of 3 feet per 
second, a mean acceleration during 0.01 second of about 15 g, and a 
natural frequency fundamental of about 15 v. p. s. M. I. T. gage III 
showed a tension strain of 0.15 percent, Williams gage 15 showed a 
reading of % pound and swinging door 4, which was in the open doorway 
facing outward, a large reading of IK inches. 

A(d) Shelter, Poured Oetober 18, 1940 

Penetration tests, November 27, 1940, at 981 feet per second 
striking velocity and 21° obliquity, with a 6-inch 100.75-pound 
projectile. The 6-inch projectile penetrated the 40K-inch solid slab 
11% inches, rebounded, and fell in front of the shelter. A close-up 
view of the penetration crater and also of the projectile is given in 
Fig. 44, and a contour sketch of the crater in Fig. 45. Two vertical 
cracks were formed on the ceiling inside the shelter, one 4 feet 
long below the axis of the explosion and the other 8 feet long, 2 feet 
to the right. Ceiling deflection was less than 0.1 inch. There was 
no indication of increased pressure on any of the blast gages. The 
Hopkinson bar showed a little flutter in one position. M. I. T. gage 
III showed an oscillating tension and compression strain of 0.01 
percent. 

Explosion test, November 27, 1940, with 22.2 pounds TNT and 0.2 
pound tetryl detonated H. O. in the projectile impact crater. The 
explosion enlarged the crater from 33 by 42 inches to 65 by 68 inches 
and deepened it from 11% inches to 13^ inches. There were a number 
of cracks extending more or less radially from the crater. The rein¬ 
forcing bars were broken and flattened back as in shelter A(c). See 
Fig. 46. Fig. 47 shows the manner of securing the TNT holder 


0 / 



Fic. -44.—Impact penetration test on Shelter A(d). Projectile 6-inch litO.TS pounds; strikins velocity 981 
feet per second; oblicpiity 21°; penetration 11^4 inches. 



Figure5 reocfo/)/e from this side 
I _ indicate depff? of xabbmg m inches 

I 


Fig. 4.5.— Contour sketch of the impact penetration crater in Shelter .\(d). 


position reenforcing dors re fa f ire undamaged 
face ffmuo rofues abore face 



































58 



Fi(i. K). Uulor roof slab of ShoUor Afd), after (ietoiiation of 22.2 pounds TNT in the impact penetra¬ 
tion crater. 



K](.. 47.—Manner of securing the TNT liolder in tlie iini)act penetration crater preparatory to detonation. 




59 


in the penetration crater. In the ceiling inside the shelter, there 
were cracks and flaking of the cement surface over an area approxi¬ 
mately 7 feet in diameter. The detonation caused a violent deflection 
of the ceiling, probably greater than 2% inches, and jerked the electrical 
leads from the gage. The gage was mangled between the inertial bar 
to which it was clamped and the concrete ceiling. The initial velocity 
of the ceiling was estimated at over 25 feet per second and the accelera¬ 
tion for 0.01 second at 100 g, which is a high acceleration over this 
time interval. Large strain values were also indicated. M. 1. T. 
gage II showed a compression strain of 0.29 percent, followed by ten¬ 
sion greater than 0.36 percent, followed by compression of 0.55 percent. 
Manganin gage 4 at the junction of the ceiling and side wall showed 
tension strain of 0.34 percent. All Hopkinson bar gages registered 
deflections beyond the scale, indicating violent motion. No significant 
blast pressure effects were registered, except on swinging door gage 6, 
which was inside the shelter near the roof, which registered a large 
deflection of 1}^ inches, probably as a result of flying particles of 
concrete. 

B(a) Shelter, Poured Oetober 12, 1940 

Penetration test, November 26, 1940, at 1,010 feet per second 
striking velocity and 23°20' obliquity, with a 3-inch 13.0-pound 
projectile. The 3-inch projectile perforated the 9-inch outer roof, 
passed through the sand fill, struck the inner slab, and lodged in the 
fill as in the case of A (a). A close-up view of the hole in the outer 
slab is given in Fig. 48, and a contour sketch of the crater in Fig. 
49. There were no cracks, scabbing or, other evidence of damage to 
the ceiling slab inside the shelter. No ceiling deflection was registered. 
The M. I. T. gages II and III showed an alternating tension and 
compression strain starting with 0.005 percent. Neither the blast 
nor Hopkinson bar gages registered. 

Explosion test, November 26, 1940, with 2.8 pounds TNT and 0.1 
pound tetryl detonated H. O. in sand between roof slabs; second 
charge, same size, December 19, 1940, detonated H. O. against top 
of inner roof slab. The explosion caused damage, less in relative 
extent though similar in character to that in the corresponding %- 
scale shelter A(a). The crater was enlarged and additional reinforc¬ 
ing steel exposed though not broken, and there were many radial 
cracks, as may be observed in the view of the outer roof slab in Fig. 
50 and in the close-up view of the crater in Fig. 51. Two vertical 
cracks appeared on the ceiling inside, one 4 feet to the right and the 
other 2 feet to the left of the explosion. It was thought that the 
reduced damage to this shelter, as compared with A(a), might have 
been due to the fact that the explosive 7>harge was not in contact with 
the inner roof slab, and on December 19, 1940, a second charge of the 




0917 / 


60 



Fig. 4S.—Closo-up view of impact penetration crater in Shelter B(a). I'rojectile 3-inch 13.0 pounds; strikinjr 
velocity 1,010 feet per second; obliquity 23°20'; penetration complete. 


- B(a) 



Fiqure3 reodaNe from f/ 1/3 3/de 
,• indicate depth of xabb/ng m inched 


% 

I 


I 

I 


I 

^ I 

§ -I 

I ^ 

S g 

B 


lo 


<30 




Fig. 49. (h)ntour sketch of impacl ])enelration crater in Shelter B(a). 




























































61 



Fig. 50.—Outer roof slab of Shelter B(a), after detonation of 2.8 pounds TXT in the sand between roof slabs. 



Fig. 51.—Close-up view of the crater in Shelter B(a). after detonation of 2.8 pounds TNT. 



62 



Ki<;. :r2. t>ulcr roof slab ol Shelter B(a), after the detonation of the second charge of 2.S|)onnds 'I'Xd' in 

the sand between roof slabs. 



Fig. 53.—Ceiling of Shelter B(a), after detonation of two charges of 2.8 pounds TNT each in the sand against 

the inner roof slab. 







63 


same size was inserted through a tube and detonated with the end in 
contact with the inner rool slab. No appreciable increase in damage 
was, however, noticeable, though two additional cracks appeared 
and there was some lengthening of the previous cracks inside the 
shelter. The total effects were not at all comparable to those in the 
K-scale model A (a). The appearance of the outer and inner slabs 
after this second explosion is shown in Figs. 52 and 53. A ceiling 
deflection of 0.10 inch 0.01 second after impact was registered. 
AI. I. T. gage II showed compression strain of 0.027 percent followed 
by tension of 0.045. M. I. T. Ill showed tension of 0.16 percent. 
Manganin gages 2 and 3 showed tension of 0.08 percent and No. 4 
showed tension of 0.03 percent. Hopkmson bar gages indicated 
measurable disturbance. A slight blast pressure was indicated on 
the swinging door gages, probably from the open doorway. 

B(b) Shelter, Poured September 25, 1940 

Penetration test, November 5, 1940, at 1,022 feet per second 
striking velocity and 20° obliquity, with a 3-inch 13.0-pound projectile. 
The action was practically identical to that observed in the %-inch 
scale model A(b). The 3-inch projectile perforated the 9-inch outer 
roof, deflected upward, struck the inner roof (penetration 1 % inches), 
and escaped through a 1-foot by 2-foot opening between roofs. 
A motion-picture record. Fig. 27, shows the projectile in flight, and the 
impact and escape of the projectile. In this case the projectile was 
]-ecovered about 30 feet distant to the rear and left of the shelter. 
Views of the roof after impact, a close-up view of the crater, and a 
view between the roof slabs are given in Figs. 54, 55, and 56. A char¬ 
acteristic elliptical hole was formed with the axis rotated in this case. 
On the ceiling inside the shelter, there was a vertical crack about 
2 feet 6 inches each side of the point of impact, reaching almost from 
top to bottom of the slab. Contour sketches of the craters in the roofs 
are given in Fig. 57, 58, and 59. The blast and bar gages gave no 
readings. The ceiling deflection gage gave no record because the 
oscillograph shutter was not opened. M. I. T. strain gage II showed 
a high alternating strain of 0.23 percent but it should not be compared 
with other readings because the gage was cemented to the surface 
and not buried in the concrete. 

Explosion test, November 27, 1940, with 2.8 pounds of TNT and 
0.1 pound tetryl detonated H. O. in the projectile impact crater on 
the inner slab. As in the case of shelter A(b), the explosion had 
negligible effect on the outer slab but enlarged the crater on the 
inner slab from 10 by 12 inches to 22 by 23 inches and deepened 
same from 1% inches to 3 inches. There was appreciable scabbing of 
the ceiling inside the shelter but neither the increased penetration 
nor the scabbing was relatively as severe as in shelter A(b). See 
Fig. 60. The Hopkinson bar and swinging door gages were knocked 


64 



Fig. 54.—Impact penetration te.^it on Shelter B(h). I’rojeetile 3-inch 13.0 pounds; striking velocity 1,022 
feet jier second; obliquity 20°; penetration outer slab complete, inner slab 134 inches. 



Fig. 55.—Close-up view of the impact penetration crater in Shelter B(b) 



65 



Fic. 56.—View i)etween roof slai)s of Shelter B(b), after the impact penetration te.st. The projectile was 
deflected by the inner slab and escaped through an opening at the top as in Shelter A(b). 








66 


Figure:) readodk from this side 
uidtcofe depth of xahb/ng in inches 




. nbi 




i 

i>V*«-Nv i 

i ''*5- 

. 

: ^ 


' t 

1 ^ 

^ 1 - o ^ ^ [• 

i ! 

. \ . . i. . 




1 I 

^ ^ k 

^ I J 




11 

Pa « 50 

§,-^ 


I 


Fk;. aT.—C'otitour sketch of the imi)act jx'nefration crater, outer fac(“ of the outer slat) in Shelter B(b). 



Figure)) reodoS/e from this side 


Fig. 58. Contour sketch of the impact penetration crater, rear face of the outer slab in Shelter 11(1'). 














67 


Figures reoM/e from this side 
indicate depth of scabbing in inches 



I 

I 

§ 

I 


P'k;. r)9. ('oTitoiir sketcli of tho impact penetration crater, outer face of the inner slat) in Slielter H(i)). 



Fig. 60.—Ceiling of Shelter B(b), showing the rather severe scabbing from detonation of 2.8 pounds TNd’ in 
the penetration crater on the outer face of the ceiling slab. 


position reenforcing dans re/afire undamaged 
face Minus values above face 


















































68 



Fic. f)!.—Close-up view of tlie impact penetration crater in Shelter B(c). Projectile :Finch 13.0 pounds; 
striking velocity 1,(>14 feet per second; oblifiuity 21°; penetration 7h inches. 



1 

I" 

I I 


iO ^ 


<5 






"I 5 -S 
^ & s 

so 

II 

^ I 




io 




Figures reodod/e from this side 
indicate depfti d xabbmg m inches 


Fig. 62.—Contour sketch of the impact penetration crater in Shelter B(c). 


















69 


around or broken and the deflection gage reading was inconclusive. 
Two of the manganin gages were also broken. M. I. T. gage 11 
showed a compression strain greater than 0.136 percent followed by 
tension of 0.15 percent followed by compression of 0.20 percent. 

B(c) Shelter, Poured October 25, 1940 

Penetration test, November 26, 1940, at 1,014 feet per second 
striking velocity and 21° obliquity, with a 3-inch 13.0-pound projectile. 
The 3-inch projectile penetrated the 15%-inch outer roof slab 7% 
inches, rebounded and dropped in front of the shelter. A close-up 
view of the crater is shown in Fig. 61 and a contour sketch of same 
in Fig. 62. There were no cracks in or scabbing of the inner slab. 
No ceiling deflection was registered, and no blast pressure or Hopkin- 
son bar readings obtained. M. I. T. gage 111 showed a slight tension 
strain of 0.007 percent and manganin gage 3 showed a compression 
strain of 0.11 percent. 

Explosion test, November 26, 1940, with 2.8 pounds of TNT and 
0.1 pound tetryl (booster) detonated H. O. in tlu' projectile impact 
crater. The explosion enlarged the crater from 20 by 20 inches to 
30 by 36 inches and deepened it from 7 % inches to 9 inches; also 
broke a number of the reinforcing bars and flattened them back 
against the concrete. See Fig. 63. There was no evidence of any 
damage to the inner ceiling slab. While there may have been cracking 
or scabbing of the outer slab, the inner surface of this slab was, of 
course, not available for inspection. The deflection gage showed a 
ceiling deflection of 0.05 inch within 0.004 second after impact. The 
initial velocity of the ceiling slab was approximately 2 feet per second, 
the mean acceleration during 0.004 second about 20 g, and the natural 
frequency about 30 vibrations per second. M. 1. T. gage II showed a 
tension strain of 0.013 percent and gage III, a tension of 0.03 percent. 
No readings were obtained on the Hopkinson bar or blast gages 
except a slight disturbance on swinging door gage 4 

B(cl) Shelter, Poured Oelober 25, 1940 

Penetration test, November 26, 1940, at 979 feet per second strik¬ 
ing velocity and 21°15' obliquity, with a 3-inch 13.0-pound projectih*. 
The 3-inch projectile penetrated the 20b-inch solid slab 6b inches, 
and as in shelter B(c), rebounded and fell in front of the shelter. A 
close-up view and contour sketch of the crater are given in Fig. 64 
and 65. There were a few short hair cracks on the ceiling inside th(‘ 
shelter. No deflection gage or Hopkinson bar readings were obtained. 
M. I. T. gage II showed a tension strain of 0.04 percent. Manganin 
gage 1 showed a compression strain of 0.10 percent, and gages 2 and 
4 tension strain of 0.05 percent. Tlie Williams gage gave a slight 
indication. 


42()r.(»4" 41-6 


70 



Fig. 63.—Close-up view of the crater in Shelter B(c) after detonation of 2.8 pounds TXT in the impact 

penetration crater. 



71 



Fig. 64.—('loso-up view of the impact penetration crater in Shelter B(d). Projectile 3-inch 13.0 pound.s; 
striking velocity 979 feet per second; obliquity 21°15': penetration 6H inches- 


B(d} 




Figure5 reoM/e from fhi5 5 ide 
indicote depth of xabb/ng m inches 


\ 

\ I 

^ -I 

1 § 

It ^ 

® « -Q 

^ J 

, 1 ''^ 

^ 1 ,^ 




Fig. 65. —Contour sketch of the impact penetration crater in Shelter B(d). 
























































Fi(i. 60.—Close-up view of the crater in Shelter B(d), after detonation of 2.8 pounds TNT in the impact 

penetration crater. 





73 


Explosion test, November 26, 1940, with 2.8 pounds TNT and 0.1 
])oiind tetrvl (booster) detonated H. O. in the projectile im])act 
crater. The ex])losion enlarged the crater from 19 by 24 inches to 
30 by 46 inches and deepened it from 6/4 inches to 7% inches; also 
l)roke and flattened back the reinforcing bars as in shelter B(c). 
See Fig. 66 . The ceiling deflection was greater than 0.75 inch. As 
in the case of shelter A(d), the deflection was violent and the initial 
velocity and acceleration of the ceiling slab were high. M. I. T. 
gage III showed a compression strain of 0.145 percent followed by 
tension of 0.03 percent. Manganin bar gage 1 showed a tension of 
0.03 percent and gage 4 a tension of 0.05 percent. All the Ho])kinson 
gages probably had deflections beyond the full scale. Swinging door 
gage 3 registered apprecial)le motion ])robably as a result of l)last 
thi-ough the open door. 

Summary of Test Results on Shelters 

A summary of the penetration results on test shelters and the 
preliminary test slab is given in Table 10. 


Table 10 . —Penetration results on test shelters 


Date of test 

Shelter 

1 

Projectile 

Thick¬ 
ness of 
outer 
roof 

Striking: 

velocity 

Obli¬ 

quity 

Penetration 

Xov. 5,1940 
Oct. 31,1940 
.\'ov. 27,1940 

Do_ 

.Vov. 26, 1940 
Xov. 5,1940 
Xov. 26, 1940 

Do_ 

Oct. 31,1940 

A (a)_ 

A(b)_ 

A(c)_ 

A(d)- 

B(a)- 

B(b)-■- 

B(c)_ 

B(cl)- - 

60" X 60" (slab)_ 

(6"-100.75#).__ 
(6"-100.75#)_._ 
(6"-100.75#) _ . _ 
(6"-100.75#) . _ 

(3"-13.0#)_ 

(3"-13.0?!f)_ 

(3"-13.0#)_ 

(3"-13.0#)__- 
(3"-13.0#)_ 

Inches 

18 

18 

31H 

40)4 

9 

9 

15*4 

20k 

16)4 

Feet per 
second 
976 
998 
1,023 
981 
1,010 
1,022 
1,014 
979 
902 

20 00 
22 00 
21 30 
21 00 
23 20 
20 00 
21 00 
21 15 
18 00 

Complete. 

Complete.i 

14)4 inches. 

Ilk inches. 
Complete. 
Complete .2 

7H inches. 

6)4 inches. 

6)4 inches. 


• 2K-inch penetration of inner roof. 
2 1^- inch penetration of inner roof. 


The preliminary test slab was detonated November 5, 1940, shelters 
B(a), B(c) and B(d) on November 26, and the other shelters on 
November 27, 1940. 

A summary of the explosion results is given in Table 11 . 































74 


Table 11 . —Explosion results on test shelters 


Shelter 

Roof 

thick¬ 

ness 

Penetration crater 

Explo¬ 

sive 

Crater after 
explosion 

Results 

Size 

Depth 

Size 

Depth 


Inches 

Inches 

Inches 

Pounds 

Inches 

Inches 



f 18 

29 X 36 

1 12 X 14 





A (a)- 

{ '22M 



22. 2 



Outer roof cracked 







bulged ; inner 








scabbed. 


f 18 

32 X 36 

' 8x 15 



... 


A(b)- 

{ 2 223 -^ 

19x22 

234 

22.2 

37 x 51 

203 ^ 

Inner roof .scabbed. 

A(c)_ 

31K 

34x 58 

143^ 

22. 2 

65 X 74 

Limit 

Cracks on ceiling. 

A(d)_ 

403^ 

33x 42 

1134 

22. 2 

65 X 68 

133^ 

Do. 


f 9 

16 X 18 

15x7 





B(a)- 




2.8 



Do. 

B(b)_ 

( 9 ’ 

16 X 18 

15x5 





{ 2 113i 

10 X 12 

134 

2.8 

22 X 23 

3 

Inner roof scabbed. 

B(c)- 


20x20 

73i 

2.8 

30 X 36 

9 

No cracks on ceiling. 

B(d)_ 

2034 

19x 24 

634 

2.8 

30 X 46 

734 

Cracks on ceiling. 

60 X 60-inch 








slab_ . 

163^ 

17 X 19 


2.8 

22 X 28 

9 



and 

roof 


> Hole. 

3 Inner roof slab. 


Summary of Instrument Results of Shelter Tests 

A summary of data obtained with the various gages, arranged for 
comparisons among structures with respect to each class of data, is 
given hereinafter. The ceiling deflections are given in Table 12. 

Table 12. —Ceiling deflections 


Struc¬ 

ture 


Thickness 


De- 

flec- 


Deflection 


Outer 

Space 

Inner 

tion 
at im¬ 
pact 

Penetration 

at 

detonation 

Notes 


Inches 

Inches 

Inches 

Inch 


Inch 


A(a)... 

18 

36 (sand) 
(earth). 

223 ^ 

0. 03 

Complete penetration 
of outer roof (projec¬ 
tile in sand between 
slabs). 


Large area of inner sur¬ 
face scabbed off; out¬ 
er roof bulged out. 

B(ai... 

9 

18 (sand).. 

1134 

None 

_Do_ 

0.10_ 

Cracks, but no scab¬ 
bing. 0.01 sec. to 
maximum. 

A(b).-. 

18 

36 (air)_ 

223^ 

0.03 

Complete penetration 
of outer roof and 234 
inches inner slab. 

0. 002 sec. to max. 


Inner surface scabbed 
off. 

B(b;... 

9 

18 (air)_ 

1134 


Complete penetration 
of outer roof and 134 
inches inner roof. 


Do. 

A (c; -. - 

313^ 

36 (sand)-- 

9 

0. 01 

1434 inches penetration. 
0. 013 sec. to max. 

0.2 (y)- — 

Short cracks inside. 
0. 01 sec. to max. 

B(c)... 

1534 

18 (sand) — 

43i 


73i inches penetration.- 

0.05 (x)-.- 

No cracks inside. 0. 004 
sec. to max. 

A(d)..- 

403i 


0 

<0.1 

1134 inches penetration 

>2.37(p) 

gauge 

broken. 

Rosette of cracks in¬ 
side 

B(d).- 

2034 


0 


634 inches penetration - 

>0.75(q) 

gauge 

broken. 

Do. 


(y) Ceiling slab, mean acceleration during 0.01 second, about 15 g. Ceiling slab, mean acceleration dur¬ 
ing 0.0005 second roughly 200 g. Ceiling slab, initial velocity 3dbl f.s. Ceiling slab, natural frequency of 
slab, fundamental about 15 v. p. s. 

(x) Ceiling slab, mean acceleration during 0.004 second, about 20 g. Ceiling slab, mean acceleration dur¬ 
ing 0.0005 second, roughly 150 g. Ceiling slab, initial velocity 2db0.5 f. s. Ceiling slab, natural frequency, 
fundamental, about .30 v. p. s. 

(pi (q) Initial velocity of ceiling evidently greater than 25 f. s. (Estimate based on deflection gage and 
Hopkinson bar deflections if & t = 0. 01 second then A .01 = 100 g and presumably 5 or 10 times this for 0.001 
second or less..) 

























































75 


Ihe porcoiitag;o of elongation })y tlio various strain gages is given in 
table 13. 


Table 13. —Percentage elongation by strain gages 




M. I 

. T. gages 


Manganin (bar) gages 


Impact 

Detonation 

Impact 

Detonation 


II 

III 

II 

III 

1 

2 

3 

4 

1 

2 

3 

4 

A (a)- 

— 

— 

— 

— 

— 

— 

— 

— 

— 

Broken. 

Broken. 

— 

B(a)_ 

f±0. 005 

±0. 005 

-0. 027 
+.045 
+.072 
-.045 

+0. 16 

0 

0 

0 

-0.03 

0 

+0.08 

fO. 08 

+0.03 

A(b)_ 

r — 


-.045 







Broken. 



\_ 









B(b)_ 

J » ±. 23 

— 

>-. 136 
+ .15 
2 













_ 

—.03 

— 1 

-. 13 

-. 11 

0 

0 

0 

0 

0 

Broken. 

0 

0 

+. 10 

Broken. 

0 

0 

0 

+.34 

A(c)- 

B(c)_ 

A(d)_ 

' -.072 
0 

±.007 

±.01 

±.013 
-.29(a) 
>.36 (b) 
>-.55(c) 

+.15 
+.03 
2 +. 11 (al) 
-. 13 (bl) 
±.015 (cl) 
-. 145 

0 

-.05 

0 

+.05 


1 _ 










B(d)_ 

1 +.04 

_ 

-. 10 

+.05 

Broken. 

+.05 

+.03 

Broken. 


+. 05 




±.03 















> For an element cemented to inner surface of wall. 

2 May have included an effect of mechanical action on cable. 

(a) Wave pulse, transverse, with inner surface in compression for about 0.001 second. 

(b) Wave pulse, transverse, with inner surface in tension (off scale) for several tenth-seconds. 

(c) Wave pulse, transverse, with inner surface in compression (off scale). 

(al) Wave pulse, transverse, with outer surface in tension for about 0.001 second. 

(bl) Wave pulse, transverse, with outer surface in compression for several tenth-seconds. 

(cl) Wave pulse, transverse, with outer surface in small ± strain oscillation. 


Increased air pressures inside the shelters by the various blast gages 
are given in table 14. 


Table 14. —Blast data 



Impact 

Detonation 

Williams 

Quartz 

S. D. 

Williams 

Quartz 

S. D. 

A (a) 




1 (7) ^lb/in.2 
(7)-(12)- 

(7)0 (12)0 _ 

(7)0 (12)0 _ 

(7)0 (12)0 
(15)3^ lb/in.2 . _ 
0 0 

1.9 lb/in.2 3 

(3) ^6" (&)%". 

(4) upset. 

(4)i.k" (5) upset. 

(3) 0 (6)0 (4) \W’. 

(4) i+" (5) 0. 

(3)0 (6) IH". 
(3)1916" (6) H". 

TUai 



(3) - (6)- 

(4) - - 

(4)- (5)- 

mi" - 

(6)0 (4)- 
(4)0 (5)0. 

(3)0 (6)0 
(3)0 (6)0.-..- 

1 1 1 

1 1 1 

\ \ \ 

1 1 1 

1 1 « 

1 1 < 

1 1 < 

1 1 

t 

(7)- (12)- 
(7)- (12)- 

(7)^lb/in.2 _ 

(12)0 (15)0 _ 

0‘ 0 


8 lb/in.2 3..... 

1.2 lb/in.2 3__ 

\(d) 

(7)0 (12)0 


(7)0 (12)0 _ 


B(d') 3 _ 

(14)^ lb/in.2_ 

(4)0 - 


(14)H lb/in.2... 
(4)H lb/in.2..„ 

0.1 lb/in.2_ 


> Numbers in parentheses indicate gage (position). 
- Effect of gun blast. 

^ Gages suspended at center of door, facing out. 













































































7G 


llopkinson bar data are given in table 15. 


Table 15 . — llopkinson bar data 




Impact 



Detonation 



I 

II 

II 

III 

I 

II 

II 

III 


Inch 

Inch 



Inch 

Inch 

- Inch 

Inch 

A (a)- 

1 0.1 

— 

— 

— 

0) 

(0 

Hr. 

(■) 

(0 

B(a)_ 

1 * 

1 — 

— 

— 

H 

H 

He 

A(b)- 


— 

— 

— 

(0 

0) 

(0 

(1) 

B(b)_ 

1 - 

1 - 

— 

— ' 

Knocked 

out. 


i 

H 

A(c)- 

0 

0 

0 

0 

0 

0 

0 ! 

20 

B(c)- 

0 

0 

0 

0 

0 

0 

0 

0 

A(d)-_ _ 

0 

0.1 

0 

— 

All gages 

had deflection>full scale 

B(d)- 





(probably all>full scale 
not in good adjustment.) 

; scriber 


> No gages used. 

2 Shock pressure in concrete<l,000 Ib./in. (“least count” for measurable deflection) (1. e. this pressure, or 
ceiling velocity of<l ft./sec. 

Note.— In the case of detonations A(d) and B(d), and probably those of A (a), B(a) and B(b), the ceiling 
slabs developed initial velocities of higher order than the order of velocity imparted to time piece of Hopkin- 
son Bar by shock pressure. Hence the large deflections of gage measure roughly the ceiling velocities. 

Supplementary Explosion Tests 

In adopting the basic bomb against which protection is to be pro¬ 
vided, the assumption was made that an AP type of bomb notwith¬ 
standing its lower explosive content, would, because of penetration 
prior to explosion, be more destructive than a light case bomb with an 
instantaneous fuse. To substantiate this assumption and to obtain 
data on the comparative destructiveness of these two types of bombs, 
the shelters with the thicker outer roofs, A(c), A(d), B(c), and B(d), 
were subjected to further tests simulating detonation without pene¬ 
tration, of 66 percent filler bombs placed against undamaged areas of 
the roof slabs half way between the existing craters and one side of the 
shelters. No instrumental readings were taken for these supplemen¬ 
tary explosion tests. 

A(c) Shelter, Supplementary Explosion Test 

December 19, 1940, with 66.6 pounds TNT and 0.4 pound tetryl 
detonated H, O. with the base in contact with an undamaged area of 
roof. The explosion formed a crater 36 by 36K inches and 6)8 inches 
deep with many radial scars on the concrete roof surface. Reinforcing- 
steel was exposed and two of the bars were broken. See Fig. 67. 
The blast caused appreciable splinter damage to the side of the adjacent 
shelter A(d). No additional cracks were formed on the ceiling inside 
the shelter. See Fig. 68. 

A(d) Shelter, Supplementary Explosion Test 

December 19, 1940, with 66.6 pounds TNT and 0.4 pound tetryl 
detonated H. O. with the base in contact with an undamaged area of 




























/ / 



Fic. 07.—Supploniontary explosion test on an undamaged area of roof of Shelter .\(c), with 60.0 pounds TNT 

detonated in contact with the roof. 



Fig. 68.—Ceiling of Shelter .\(c) after the impact penetration test followed by detonation of 22.2 pounds 
TNT; also detonation of 00.6 pounds TNT in contact with an undamaged area of the roof halfway between 
the impact penetration crater and the side of the shelter. 













78 



Fn.. ')9.-Supploinontary explosion test on an undamaged area of roof of Shelter A((l), with 66.6 pounds 

TNT detonated in contact with the roof. 








79 


roof. The explosion formed a typical explosion crater like the one in 
sheltei A(c), 36 by 33 inches and 6% inches deep. The reinforcing steel 
was somewhat more damaged than in shelter A(c). See Fig. 69. 
A considerable number of splinter craters were made on the side of 
shelter A(c), some reaching to the reinforcing steel. A diagonal corner 
crack was formed on the ceiling inside the shelter opposite the explo¬ 
sion. Fig. 70 shows a view of the ceiling after both explosions. 

B(c) Shelter, Supplementary Explosion Test 

December 19, 1940, with 8.4 pounds TNT and 0.2 pound tetryl 
detonated H. O. with the base in contact with an undamaged area of 
roof. The explosion produced characteristic results, the crater formed 
being 15 by 16 inches and 4% inches deep. The reinforcing steel was 
exposed to some extent but not broken. See Fig. 71. Again per¬ 
ceptible splinter damage was done to the adjacent shelter B(d), several 
holes being 3 inches in diameter and % inch deep. No cracks were 
formed on the ceiling inside the shelter. See Fig. 72. 

B(d) Shelter, Supplementary Explosion Test 

December 19, 1940, with 8.4 pounds TNT and 0.2 pound tetryl 
detonated H. O. with the base in contact with an undamaged area of 
roof. A typical explosion crater was formed 14^ by 13K inches and 
3^6 inches deep. The reinforcing steel was not broken. See Fig. 73 . 
There was some damage to the side of the adjacent shelter B(c). 
Additional hair cracks were formed on the ceiling inside. Fig. 74 
shows a view of the ceiling after both explosions. 

Summary of Supplementary Explosion Tests 

A summary of the supplementary explosion tests is given in Table 16. 


Table 16. —Supplementary explosion tests 


Shelter 

Slab 

thick¬ 

ness 

Explo¬ 

sive 

Crater size 

Penetration and remarks 


Inches 

Pounds 

Inches 


A(c)_ 

31H 

66.6 

36 X 36% 

6% inches; no additional ceiling cracks. 

A(d)- 

401-^ 

66.6 

36x33 

6% inches; additional diagonal ceiling crack. 

B(c)_ 

15% 

8.4 

15 X 16 

4% inches; no ceiling cracks. 

B(d)- 

20% 

8.4 

14% X 13% 

3%6 inches; additional hair cracks in ceiling. 


ADDITIONAL TESTS ON SLABS 

The results of the tests on the shelters indicated the need for addi¬ 
tional penetration and explosion tests to supplement the data ob¬ 
tained, and also to experiment with arrangements of reinforcing steel 
other than the conventional two-way reinforcing, which had not 
appeared to advantage in the shelter tests. Special reinforcing was 

















80 



Fio. 71 .—Supplementary explosion test on an undamaged area of roof of Shelter B(c) with 8.4 pounds T X^l’ 

detonated in contact with the roof. 



Fig. 72.—Ceiling of shelter B(c) after the supplementary explosion test and the prior impact and explosion. 











81 



Fic. 73.—Supplementary explosion test on an undamaged area of roof of Shelter B(d), with 8.4 pounds TNT 

detonated in contact with the roof. 







82 


designed to meet the requirements developed in the shelter tests, and 
eight additional slabs were poured on January 13, 1941, all with the 
special reinforcing and four of the eight with steel antiscabbing 
plates on the bottom or rear faces. 

Description of Slabs 

The slabs were 5 feet square. Four of the slabs were 9 inches thick 
and four 11){ inches thick, representing respectively 4- and 5-foot 
thick slabs in the prototype. Two slabs of each thickness were con¬ 
structed with a }^-inch mild steel plate on the bottom, well anchored 
to the concrete by }^-inch anchors 7 inches long spaced 8 inches centers 
both ways, welded to the plates. The shelter tests had indicated 
the need of more shear reinforcing and of attaching it securely to the 
longitudinal and transverse steel in the faces. No greater percentage 
or total amount of steel was provided in the revised design, but it was 
disposed with the above objective in view, more in the web and less 
in the faces. In addition, the transverse and longitudinal spacing 
was made the same. The longitudinal steel was arranged in the 
form of a truss with the diagonal shear steel welded to the chord steel. 
The transverse reinforcing was threaded through, inside the transverse 
chord steel at each face. The details are shown in Figs. 75, 76, 
and 77. 

The same reinforcing steel, concrete aggregates and cement were 
used as in the shelters. The mix consisted of Sji cubic feet of gravel 
(1 percent moisture), 7 cubic feet of sand (3 percent moisture), 3 bags 
of ‘Tncor’’ high early strength cement, and 13 gallons of water. The 
mixing time was Iji minutes and the slump was 4}{ inches. The 
aggregates were heated. Test cylinders taken on January 13, 1941, 
and tested January 23, 1941, showed a compressive strength of 4,010 
pounds per square inch, a strain at 1,500 pounds per square inch of 
414 millionths of an inch per inch, and a weight per cubic foot of 139 
pounds. The workmanship and curing appeared satisfactory though 
there were perceptible shrinkage cracks parallel to the trusses and 
some honeycomb on the bottom faces. 

Test Conditions 

Four of the slabs, one of each type for the penetration tests, were 
supported on heavy timber A-frames with the faces tilted back to 
give an obliquity of impact of approximately 19°. See Fig. 78. 
Sand was filled in between the A-frames and rear faces of the slabs. 
As the slabs had been constructed at scale, the 3-inch gun and pro¬ 
jectiles were used. The projectiles were 3-inch uncapped A. P. 
projectiles, weighing 13.75 pounds. The other four duplicate slabs 
were supported in a horizontal position on heavy timber box frames 
filled with sand, and were subjected to untamped explosive charges 


83 



Fig. 75.—Details of test slabs and reinforring. 
























































































































































84 










- 










'•v;-iY,r.iitOTiiiin<i-<^’*’- 




mLi... 


10 ummmmmmiJmlti 




Fig. 77.—SiM'cial reinforcing for lH4-inch additional test slabs. 













































85 




420504°—41 


Mounting for impact penetration tests of additional slabs. Sand was filled between the wood A-frames and the rear faces of the slabs 







86 


of 8.4 pounds TNT, simulating detonation of a 66 percent fdler light 
case bomb on the surface of each slab. The penetration tests were 
made on February 19, 1941. 

Penetration Tests 

C(al) slab, 9 inches thick, at 873 feet per second striking velocity 
and 19° obliquity. The 3-inch projectile perforated the 9-inch slab 
with appreciable residual velocity. The crater on the front face was 
approximately 18 inches wide by 20 inches high. See Fig. 79. A 
through crack formed in the direction of the transverse reinforcing 
from the crater to the top of the slab. The scabbing crater, 25 by 
19 inches, on the rear face is shown in Fig. 80. 

C(bl) slab, 9 inches thick with }^-inch steel plate attached to the 
rear face, at 988 feet per second striking velocity and 19° obliquity. 
The 3-inch projectile perforated the 9-inch slab and the %-inch steel 
plate on the back, with noticeable residual velocity. The crater on 
the front face was approximately 18 inches wide by 21 inches high. 
See Fig. 81. The perforation formed a 4-inch bulge and 4-inch spur 
of the steel plate on the rear face as may be noted on Fig. 82. 

C(cl) slab, 11)^ inches thick, at 977 feet per second striking velocity 
and 19° obliquity. The 3-inch projectile perforated the ll)^-inch slab 
with some residual velocity. Fig. 83 shows the 21- by 18-inch 
crater on the front face and Fig. 84 the 25- by 26-inch scabbing 
crater on the rear face. 

C(dl) slab, 11}^ inches thick with }^-inch steel plate attached to the 
rear face at 968 feet per second striking velocity and 19° obliquity. 
The 3-inch projectile penetrated the ll}^inch slab, deflected upward 
and to the right, and lodged against the }^-inch steel plate. A crater 
17 inches wide by 18 inches high was formed on the front face. See 
Fig. 85. The 2-inch bulge formed on the steel plate is shown in 
Fig. 86. 

Explosion Tests 

The explosion tests on the corresponding series of additional slabs 
were also made on February 19, 1941. Charges of 8.4 pounds of 
cast TNT in cylindrical containers of steel pipe were detonated in 
end contact with the slabs. Fig. 87 is a view of one of the charges 
in position for static detonation. 

C(a2) slab, 9 inches thick. The explosion formed a crater approxi¬ 
mately 18K inches in diameter and 3^ inches deep on the upper front 
face and a scabbing crater 22 by 20 inches and 1 }^ inches deep on the 
rear face. See Figs. 88 and 89. Radial cracks from the crater to 
the center of each edge of the slab divided the slab in quarters. 

C(b2) slab, 9 inches thick with }^-inch steel plate attached to the 
rear face. The explosion formed a crater 18K inches in diameter and 


87 



Fig. 79.—lnii)act iicnetration t<'st of 9-inch Slab Cfal). l’roj(‘ctiI(> 3-inch 13.75 jwund.s; striking vc'Iocity 
S73 feet i)er second; obliquity 19°: penetration complete. 



Fig. 80—View of rear face of Slab C(al) after ix'rforation by 3-inch projectile. 








88 



Fig. 81.—Impact penetration test of 9-inch Slab C(hl), with J-Hnch steel plate attached to rear face. Pro¬ 
jectile 3-inch 13.75 pounds; striking velocity 988 feet per second; obliquity 19°; penetration complete. 



Fig. 82.—View of rear face of Slab C(bl) after perforation by 3-inch projectile. 












89 



Fig. 83.—Impact penetration lest of llH-inch Slab C(cl). Projectile 3-inch 13.75 pounds; striking velocity 
977 feet per second; obliquity 19°; penetration complete. 



Fig. 84.—View of rear face of Slab C(cl), after perforation by 3-inch projectile. 



90 



Fig. 85.—Impact ponotratioii tost of 1134-inch Slab C(dl) with '^-inch sti'ol plate attached to rear face. 
I’rojectile 3-inch 13.75 pounds; striking velocity 968 feet per second; obliquity 19°; penetration complete 
through concrete, projectile lodged against the steel plate. 



Fig. 86.—Rear face of Slab C(dl), after impact penetration test, showing the 2-inch bulge formed in the 

steel plate by the projectile. 






91 



Fig. 87.—View of 8.4 pounds TNT in position for static detonation against the additional test slabs. The 
sand filled wood-frame supports are also shown. 

4% inches deep on the upper face and a 2)^-inch bulge in the steel plate 
on the rear face; also cracks radiating from the crater to the center 
of each edge of the slab. See Figs. 90 and 91. 

C(c2) slab, 11)^ inches thick. The explosion formed a crater 20 
inches in diameter and 5 inches deep on the front face and a scabbing 
area 16 inches by 20 inches on the rear face over which the concrete 
was disintegrated but not rejected. See Figs. 92 and 93. Cracks 
extended radially from the crater to the center of each edge of the 
slab. The shattering of the edges of the slab was caused by the 
splinters and concrete fragments from explosions on adjacent slabs. 
As this slab was nearly but not quite perforated by the explosion, the 
result appeared suitable for use in calibrating the explosion penetration 

formula S—ac' \jC. 

C(d2) slab, 11)4 inches thick with %-inch steel plate attached to the 
rear face. The explosion formed a crater 23 inches in diameter and 
4K inches deep on the front face and a 1-inch bulge of the steel plate 
on the rear face. See Figs. 94 and 95. Crackswere formed radiating 
from the crater to the center of each edge of the slab. 




92 



P"iG. 88.—Upper face of 9-inch Slab C(a2) after detonation of 8.4 ])ounds TNT. 



Fig. 89.—Scabbing on rear face of Slab C(a2) from detonation of 8.4 pounds TNT on upper face. Nol(' 

how the special n'inforcing reduces the scabbing area. 





93 



Fig. 90. -T’ppcr face of 9-incli Slab ('(b2) with 'H-inch stei'l i>lat(' attaclied to rear face, after detonalion of 

8.-1 ])ounds 'TXT. 



Fig. 91.—Rear face of Slab C(b2) after detonation of 8.4 pounds TXT on the upper face, showing the 2H-inch 

bulge formed in the steel plate. 






94 



Fig. 92.■—Upper face of llH-inch Slab r(c2) afti'r dotonation of 8.4 pounds 



Fig. 93.—Rear face of 111.4 inch Slab C(c2) after detonation of 8.4 pounds TNT. As this slab was not (piite 
perforated by the explosion, the result was use<l in calibrating the explosion penetration formula 


S=ac' yc 





95 



Fig. 94.—Upper face of lU4-inch Slab Cfd'J) with i-s-mch steel ])late attached to rear face, after detona¬ 
tion of 8.4 pounds TXT. 



1 -iuch bulge formed in the steel plate. 








96 


Siininiary of AcIcUlional Test Hesults on Slabs 

A summary of the additional impact penetration results on slabs 
is given in Table 17. 


Table 17 .—Additional impact penetration results on slabs 


Slab 

Slab 

striking 

velocity 

Obliqui¬ 

Crater size 

Penetration and remarks 

thickness 

ty 

Front 

Rear 


Inch es 

F. s. 

0 

Inches 

Inche.^ 


Cfal)_ 

9 

873 

19 

18 X 20 

25x 19... 

CompFde; through crack to top of 
slab. 

C(bl)..... 

1 9 

988 

19 

18 X 21 

4-inch hole in 
steel. 

Complete; through slab and steel 
plate. 

C(cl)_ 

ilH 

977 

19 

21 X 18 

25 X 26 . _ 

Complete. 

Through the concrete; 2-inch 
bulee in steel plate. 

C(di) _ 

1 iiH 

968 

19 

17 X 18 



1 With H inch mild steel plate attached to rear face. 


A summary of the additional explosion results on slabs with 8.4- 
pound charges of cast TNT is given in Table 18. 


Table 18 .—Additional explosion results on slabs 


Slab 

Slab 

thickness 

Crater size 

Penetration and remarks 

Front 

(diameter) 

Rear 

C(a2) _ 

Inches 

9 

1 9 

im 

1 10/4 

Inches 

18^2 

im 

20 

23 

Inches 

22 X 20 

inches in front; Yi inches in rear; radial cracks. 

4K inches in front; 2J-^ inch bulge in steel plate; radial 
cracks. 

5 inches in front; concrete over scabbing area disintegrated, 
and loose; radial cracks 

AY inches in front; 1 inch bulge in steel plate; radial cracks. 

C(b2)_ 

C(c2)_ 

16x 20 

C(d2)_ 




1 With yi inch mild steel plate attached to rear face. 


FLKTllEK PENETRATION TESTS ON SLABS 
Description of Slabs 

To obtain confirmatory data on the effect of the special reinforcing 
arrangement and antiscabbing plates on penetration and scabbing, 
and also to determine, if practicable, a limit slab thickness for the 
basic bomb selected, six additional slabs 5 feet square, similar to the 
other test slabs, were poured on April 3, 1941. Two of the slabs 
were 9 inches thick, two 11^ inches thick, and two ISji inches thick, 
representing again at Yu scale, slab thicknesses in the prototype 
structures of 4, 5, and 6 feet, respectively. Truss type of reinforcing 
steel was again used. Mild steel antiscabbing plates were provided 
on the ll)i-inch slabs to permit comparison with the 13K-inch slabs 
without antiscabbing plates, or, in other words, to determine the 
comparative value in the prototype of the antiscabbing plate or an 
extra foot thickness of concrete. The same materials as in the 





































97 


previous slabs were used. The mix consisted of 9)2 cubic feet of gravel 
(Iji percent moisture), 7 cubic feet of sand (5)^ percent moisture), 
3 bags of ^‘Incor’’ cement, and 8)2 gallons of water. The mixing time 
was l}i minutes, and the slump was 3% inches. The test cylinders 
taken April 3, 1941, and broken April 10 , 1941, showed a compressive 
strength of 3,080 pounds per square inch, a strain at 1,500 pounds 
per square inch of 485 millionths of an inch per inch, and a weight 
per cubic foot of 144 pounds. During curing, shrinkage cracks 
appeared parallel to the trusses. The same A-frames, projectiles, and 
gun were used as in the previous slab tests. The penetration tests 
were made on May 2 , 1941. 

Penetration Tests 

D(al) slab, 9 inches thick, at 698 feet per second striking velocity 
and 18°30' obliquity. The projectile perforated the 9-inch slab but 
stuck in the rear face, projecting 1% inches. During penetration it 
deflected both upward and to the right at an angle of 21 °. The impact 
crater on the front face was approximately 14 inches wide by 16 
inches high. A scab approximately 29 inches wide by 24 inches high 
remained loosely attached to the rear face of the slab. See Figs. 
96 and 97. 

D(a 2 ) slab, 9 inches thick, at 680 feet per second (estimated) 
striking velocity and 20 ° 10 ' obliquity. The projectile penetrated 
only 4% inches, rebounded, and came to rest in front of the slab. 
The impact crater was approximately 12K inches wide by 16 inches 
high. No velocity reading was obtained for this shot, due to a 
premature breaking of the gun muzzle wire. The shallow penetration 
and appearance of the impact crater indicated that the projectile may 
have struck with a yaw, which would be possible at such low velocity. 
There were scabbing cracks on the rear face over an area approxi¬ 
mately 18 by 22 inches. See Figs. 98 and 99. 

D(bl) slab, 11 }^ inches thick with )^-inch steel plate, at 954 feet 
per second striking velocity and 19°10' obliquity. The projectile 
penetrated the ll}^-inch slab, deflected upward and slightly to the 
left, and lodged against the antiscabbing plate. The impact crater 
measured approximately 17 inches wide by 18 inches high. The 
antiscabbing plate on the rear face was bulged outward 1 }^ inches. 
See Figs. 100 and 101 . 

D(b 2 ) slab, 11}^ inches thick with }^-inch steel plate, at 1,007 feet 
per second striking velocity and 18°30' obliquity. The projectile 
penetrated the slab, deflected upward and to the right, and lodged 
against the antiscabbing plate. The impact crater was 22 inches 
wide by 19 inches high. The center of the rear face of the projectile 
was 8 K inches in from the front face of the slab. The antiscabbing 


98 



Fig. 96.- Inii)act penetration test of9-incli Slab Dtal). Projectile 3-inch 13.75 pounds; striking velocity698 
feet per second; obliquity 18°30': penetration complete. 



Fig. 97.—Hear face of Slab D(al) after impact penetration test showing the 3-inch projectile projecting 

1?4 inches. 









99 



P i(i. 98.- Inij^act i)en('tration test of 9-inch Slab l)(a2). Projectile 3-inch 13.75 pounds; striking velocity 
675 feet per second (estimated); obliquity 20°10'; penetration 4% inches. 



Fig. 99.—Scabbing on rear face of Slab D(a2) from impact penetration, 










100 



Fig. 100.—Impact penetration test of ll}.4-inch Slab D(bl) with i-^-inch steel plate attached to the rear face. 
Projectile 3-inch 13.T.*) pounds; striking velocity 9.'>4 feet per second; obliquity 19°10': penetration complete, 
projectile lodged against the steel plate. 












101 


plate on the rear face was bulged outward 1% inches. See Figs. 
102 and 103. 

D(cl) slab, 13)2 inches thick, at 991 feet per second striking velocity 
and 20°35' obliquity. The projectile penetrated 8% inches, re¬ 
bounded slightly, and came to rest against a reinforcing bar. The 
impact crater measured approximately 25 inches wide by 26 inches 
high. There was a scabbing area on the rear face approximately 
21 inches wide by 24 inches high about half the area remaining in 
place, but loose. See Figs. 104 and 105. 

D(c2) slab, 13K inches thick, at 1,001 feet per second striking veloc¬ 
ity and 19°10' obliquity. The projectile penetrated into the slab, 
deflected upward and slightly to the left, and stuck with the center of 
the rear face of the projectile 3% inches from the front face of the slab. 
The impact crater was approximately 17 inches wide by 19^ inches 
high. There was decided scabbing over an area on the rear face 
23K by 25 inches. The concrete broke up into squares over the rein¬ 
forcing and remained in place though loose. There was a horizontal 
crack entirely across the slab through the impact zone, in the direction 
of the transverse reinforcing steel. See Figs. 106 and 107. 

Summary of Further Penetration Tests on Slabs 


A summary of the further impact penetration results on slabs is 
given in Table 19. 

Table 19. —Further impact penetration results on slabs 


Slab 

Slab , 
thick¬ 
ness 1 

striking: 

Obliq¬ 

Crater size 

Penetration and remarks 

\elocity 

uity 

Front 

Rear 


Inches I 

F. s. 

0 / 

Inches 

Inches 


I)(al)_ 

9 

698 

18 30 

14 X 16 

29x 24 

Complete; projectile stuck in rear face. 

D(a2)_ 

9 1 

1 680 

20 10 

12^^x 16 


4% inches; velocity reading not ob¬ 
tained; result inconclusive. 

D(bl)- 

2 1U4 

954 

19 10 

17 X 18 

18 X 22 

Through the concrete; inches bulge 

in steel plate. 

D(b2)_ 

»11^1 

1.007 

18 30 

22 X 19 


Through the concrete; 1^4 inches bulge 
in steel plate. 

r>(cl)_ 

13 

991 

20 35 

25 X 26 

21 X 24 

8% inches; projectile rebounded. 

D(c2)- 

13H> 

1,001 

19 10 

17 X 19^^ 

2314 X 25 

Almost complete; projectile stuck in 
the concrete. 


> Estimated. 

- With i-i-inch mild steel plate attached to rear face. 


Penetration and Explosion Test 

To obtain data on the destruction resulting from the detonation of 
a bomb which has penetrated into but not entirely through the outer 
roof slab, with an antiscabbing plate fastened to the rear face similar 
to the slabs shown in Figs. 85, 100, and 102, a 2.8-pound charge of 
TNT, simulating the 20 percent filler of the basic bomb, was detonated 
at the point of maximum penetration against the plate, with the results 
shown in Figs. 108 and 109. The hole measured approximately 

420504°—41——8 




















102 



Fig. 102.—Impact penetration test of llH-inch Slab D(b2) with >4-inch steel jilate attached to the rear face. 
Projectile 3-inch 13.75 pounds; striking velocity 1,007 feet per second; obliquity 18°30'; penetration 
complete, projectile lodged against the steel plate. 



Fig. 103.—liear face of Slab D(b2) showing l^i-inch bulge formed in the steel plate by the impact penetration. 


















103 





Fig. 104.—Inijiact penetration test of 13H-inch Slab D(cl). Projectile 3-inch 13.75 pounds; striking velocity 
991 feet per second; obliquity 20°35'; penetration 8% inches. 



Fig. 105.—Scabbing on rear face of Slab D(cl) from impact penetration. 













104 



Fig. lOfi. Impact iionotration test of ISJ^'^-inch Slab D(c2). Projectile 3-incli 13.75 pounds; striking velocity 
1,001 feet per second; obliquity 19°10'; jicnetration almost complete. 



Fig. 107.—Scabbing on rear face of Slab D(c2) showing how the special reinforcing limits the scabbing arc 

and tends to retain the shattered concrete. 







105 





Fig. 108 . 



Fig. 109.—Damage resulting from detonation of a 2.8-pound charge of TNT at point of maximum penetra¬ 
tion against the J-i-inch steel plate attached to the rear face of the slab. 









106 


17 inches wide by 14 inches high. The K-inch plate was stripped off 
the rear face of the slab and the damage was generally severe, as may 
be noted. 

TAMPED EXPLOSION TESTS ON SHELTERS 

Additional tests were required to obtain data on earth pressures 
and shock against shelter floors and side walls below ground from 
tamped explosions of bombs which penetrate adjacent to or under the 
shelters. As in other tests in the program, the conditioning selected 
was that which would be the most destructive to the test shelters. 
For these particular tests, explosive charges were selected simulating 
2,000-pound bombs of the light case type containing a maximum 
practicable percentage of explosive. It was assumed that this type of 
bomb would be able to penetrate to considerable depths in average 
earth without breaking up. 

Positions and Amounts of Explosives Charges 

The explosive charges were to be placed under the shelters in a 
plane vertically through the center of the shelters and in the positions 
and amounts shown in Fig. 110. Assuming earth pressures from 
underground explosions to be similar in their effects to static pres¬ 
sures, it can be shown that to produce identical strains, the unit 
earth pressures on the under surface of the shelters should be the 
same in models and prototype. The amount of explosive used in 
the tests corresponds approximately to the filler of a light case bomb 
of a 65 percent charge-weight ratio, and for the two scales of models, 
varies approximately as the cube root of the model scale ratio. For 
the %-scale, the charge was 66.6 pounds and for the Ke-scale, 8.4 
pounds. The unit earth pressure at any distance from the center of 
explosion, is thought to vary directly as the amount of explosive, and 
inversely as the radius of camouflet times the square of the given 
distance. At the rim of the camouflet, accordingly, it would vary 
inversely as the cube of the camouflet radius. The radius of camouflet 
varies as the cube root of the explosive charge. The character of the 
soil was somewhat variable over the site, but in general consisted of 
sandy silt and clay with an average of 46 percent sand, 29 percent silt, 
and 25 percent clay. The calculated radius of camouflet for this soil 
and for the 66.6-pound charge for the %-scale models was approxi¬ 
mately 12 feet, and the radius for the 8.4-pound charge for the 
Ke-scale models, approximately 6 feet. For the tests on models A (a), 
A(b), B(a), and B(b), the charges were to be placed at these respective 
depths, and, it was expected, would result in camoufiets, with possibly 
some rupture, producing the same unit pressures on the undersides 
of both scale models, and therefore because of the relative weights 
and bottom areas, twice the upward displacement of the Ke-scale 


107 



Fig. 110.—Positions and amounts of explosive used in tamped explosion tests. 


models as for the %-scale models. For the tests on models A(c), A(d), 
B(c), and B(d), charges were placed at two-thh*ds the corresponding 
depths of the first set. It was expected that craters would result in 
these cases, and also increased pressures and displacements from 
these nearer charges. 


Tamped Explosion Tests 
Test on shelter B(c), July 31, 1941 

Instruments .—The instruments used for measuring pressures and 
displacements in the first shelter tested, Shelter B(c), were as follows: 
Two resistance type pressure gages; 

One strain-gage type accelerometer; 

Two electrically recording displacement gages; and 
One mechanical displacement gage. 

The pressure gage consisted of a manganin wire placed in a rubber 
bulb about 3 inches in diameter, and surrounded with oil. One of 
the pressure gages was placed directly above the charge and in 
contact with the bottom of the shelter, while the other gage was 
placed within the gi'ound at a radial distance of approximately 3 feet 
6 inches from the charge and 2 feet below ground level. The lead 
cables of both gages were carried first into the shelter through drilled 
holes in the bottom slab, thence to the Instrument Shelter and con¬ 
nected to cathode ray oscillographs. 
























108 


The accelerometer was a modified AIIT strain gage, consisting of a 
U bar with strain gage elements fastened to a bridging piece between 
the ends. Welded to another U bar serving as base, the accelerometer 
was bolted to the floor slab, close to the pressure gage above the 
charge, and connected by cable to an oscillograph in the Instrument 
Shelter. The bending of the bar and hence the change of resistance 
of the strain gage is proportional to the acceleration of the end at¬ 
tached to the floor. Both accelerometer and pressure gage readings 
were recorded by a cathode ray oscillograph. A view of the original 
accelerometer is shown in Fig. 112 and the remodeled type in 
Figs. Ill and 121. 

The two electrical displacement gages, placed approximately 1 inch 
apart, alongside the accelerometer, consisted of two telescoping 
bakelite shafts, with the outer shaft or sleeve about I 2 inch in diameter 
and 24 inches long, and the inner tube 12 inches long. See Figs. 
Ill and 112. Both gages were connected by wires to the oscillo¬ 
graphs in the Proof Room. 

Except for the wiring, the mechanical displacement gage differed 
but little from the electrical displacement gages. The sleeve of this 
gage was adjusted to register a maximum vertical displacement of 
4 inches. The purpose of the mechanical gage was to serve as a 
check upon the accuracy of readings to be obtained from the electrical 
gages. 

All five gages were placed along a transverse line located 5 feet 
3 inches from the front wall approximately on the center line of the 
contact surface or ground intersection plane of the inclined bottom 
slab. The bottom ends of the displacement shafts were in contact 
with the inner face of the bottom slab while the tops were braced 
against a 5-inch rail section about 5 feet long, suspended from the 
ceiling and side walls in the manner of a counterweight. See Fig. 
112 . 

Explosive charge .—A trench approximately 18 inches wide, and 
extending from the foreground inward to a distance about 5 feet, 
located midway between the two ends of the shelter, was dug to a 
depth of 4 feet 3 inches below ground line. See Fig. 113. The charge 
consisting of 8.4 pounds of TNT plus 0.2 pound of tetryl booster, in 
a metal container, was placed at the bottom of the trench at its inner 
end in a position paralleling the inclined plane of the wall, the nose 
of the container pointing inward and down. The second pressure 
gage was placed during backfilling at about mid-depth of the trench. 
After the trench was backfilled and each layer of the damp earth 
thoroughly tamped, the charge was detonated. 

Explosion damage .—As expected, the explosion residted in a crater, 
rather than a camouflet, with an opening directly below the front 
edge of the shelter. See Fig. 114. Due to the obviously large propa- 


109 




PIEZO-ELECTRIC PRESSURE GAUGE 


DISPLACEMENT 

GAUGE 



STRAIN GAUGE ACCELEROMETER 



EDDY CURRENT 
ACCELEROMETER 


Fig. 111.—Types of instruments used in tamped explosion tests. 



Fig. 112.—Shelter B(c) showing instruments used: (1) two electrically recording resistance gages, (2) strain 
gage accelerometer. (Two electric resistance type pressure gages were buried in the earth under the 
shelter). 




























































































110 




Fig. 113.—Explosive charge of 8.4 pounds of TNT in position under Shelter B(c). 








Ill 



Fig. 114.—Crater and cracks formed by tamped explosion of 8.4 pounds of TNT, 4 feet under Shelter B(c) 



Fig. 115.—Showing the destruction of instruments and the floor of Shelter B(c). by tamped explosion of 8.4 
pounds of TXT 4 feet under the floor directly under the point marked “X”. 




112 


gated pressure wave, the bottom slab was badly cracked and the 
accelerometer and all three displacement gages shattered, as may be 
noted in Fig. 115. The manganin wire pressure gage reading exceeded 
the scale of calibration and a value was estimated by extrapolation 
between 2,000 and 3,000 pounds per square inch. No other instru¬ 
ment readings were obtained. There was visible cleavage between 
the ground and the shelter in side elevation, indicating either vertical 
displacement of the shelter or upheaval of the surrounding earth. 
Part of the ejected earth from the explosion crater was packed solidly 
under the front edge of the shelter, at two sides of the hole, in the 
form of triangular wedges, while the remainder was heaped in front 
or scattered around. The explosion crater in the soil measured ap¬ 
proximately 6 feet frontwise, had a maximum depth of 4 feet, and 
extended transversely 5 feet 3 inches. There was also some damage 
to the underside or contact face of the bottom slab in the form of a 
crater measuring 14 inches transversely, 20 inches longitudinally, and 
1)2 inches deep. 


Test on Shelter A(c), August 7, 1941 

Instruments ,—Due to the inconclusive readings of the preceding 
test, the following changes were made in the instrumentation: first, 
the strain gage elements of the accelerometer were moved from the 
ends to the upper inside corner of the U bar and, with a view of further 
decreasing the anticipated strain, the insti-ument was set at the 
inner left-hand corner of the shelter, as indicated in Fig. 117; second, 
a quartz crystal piezo-electric type of pressure gage, as illustrated 
in Fig. Ill, was substituted for the bulb-inclosed manganin wire and 
installed under the floor slab near the left-hand corner through a 
side trench; third, a second pressure gage of the crusher type was 
placed under the slab directly above the charge, intended to serve 
as an approximate check upon the readings of the quartz pressure 
gage; and last, the displacement gages were omitted. 

Explosive charge .—The charge, consisting of 66.6 pounds of TNT in 
a cylindrical container 10 inches in diameter and 24 inches long, was 
placed in the trench directly below the mid-point of the front inter¬ 
section line between shelter and ground, at a depth of about 7 feet 
3 inches. The backfill, which was somewhat drier in comparison 
with the fill for Shelter B (c) in the preceding test, was placed in layers 
and thoroughly tamped. 

Explosion damage .—Explosion effects were comparable to those of 
Shelter B{c). Here, again, no camouflet was formed and the most of 
the ejected earth from the crater was packed under the ledge of the 
shelter. See Fig. 116. The crater was in the form of an inclined 
shaft about 12 feet deep, the shallowest point being approximately 
8 feet 6 inches below grade vertically, and the opening at the top 


113 


V 



Fig. 116.—Crater and cracks formed by tainiied explosion of 66.6 pounds of TNT. 7 feet 3 inches under 

Shelter .\.(c). 



Fig. 117.—Showing dania.ge to the floor of Shelter .Vfc) by tamped explosion of 66.6 pounds of TNT, 7 feet 3 

inches under the shelter. 





114 


measured about 7 feet frontwise. There was no crater in the con¬ 
crete on the underside of the floor slab in this shelter. Fig. 117 
shows the severe damage to the floor of the shelter. No instrument 
readings were obtained in this test. Fig. 118 shows an end view of 
the cleavage between the rear wall and backfill. The wedge-shaped 
opening with a maximum clearance of 9 inches at the top indicated 
a drop of the front end of the shelter with respect to the rear. 
Doubtless due to the propagated pressure wave, ground cracks 
appeared within a radius of 50 feet from the shelter. Fig. 119 
shows cracks formed on the rear face of the shelter. 

Test on Shelter B(d), August 14, 1941 

Instruments .—The accelerometer and piezo-electric pressure gage, 
as previously described, were the only two instruments used for this 
test. Both were installed in the upper left-hand corner of the floor, 
the former being located 2 feet 4 inches from the rear wall and 3 feet 
6 inches from the east side wall, while the hole for the latter was located 
2 feet 6 inches from the rear wall and 1 foot 10 inches from the east 
side wall. To obtain data on the vertical displacement of the shelter, 
motion pictures (24 frames per second) were taken and also a still 
picture with the shutter open during the explosion. 

Exvlosive charge .—The depth of the 8.4 pound charge, measured to 
the center of the container in the trench, was 4 feet 6 inches. The 
charge was placed midway between the two ends of the shelter and 
transversely 3 feet 6 inches from the front, approximately in a vertical 
plane passing through the intersection line of the floor with the ground 
line. The trench was filled in layers and the fill thoroughly tamped. 

Explosion damage .—The explosion formed a crater. See Fig. 120. 
While the inclined axis of the crater measured about 7 feet 6 inches 
the vertical depth was no more than the original depth of the trench. 
The transverse width at the top was about 2 feet 9 inches. No damage 
to the underside of the floor slab was observed, but in the inside face 
there were two typical longitudinal cracks, as may be noted in Fig. 
121. There was no apparent damage to the inside face of the rear 
wall, but a diagonal crack at each end could be observed in the outside 
face. A small pressure reading was obtained but has not been recorded 
because the gage was too far away and was not facing the charge. 
Fig. 122 shows the cleavage between the earth behind the shelter and 
the rear wall. Fig. 123 was taken with the shutter open during the 
explosion. The shadowy double exposure above the shelter and the 
dark line above the bent bar leaning against the rear of the shelter, 
show approximately the vertical displacement of the shelter. The 
amount of the displacement as determined by projecting the motion 
pictures of the shelter, taken during the explosion, on a ruled screen, 
was 8.2 inches at the front and 5.9 inches at the rear. 


115 



Fig. 118.—Separation of earth behind Shelter A(c) from the vertical displacement as a result of the tamped 

explosion. 



Fig. 119.—Cracks formed on the rear shelter face by tamped explosion of 66.6 pounds of TNT under Shelter 

.\(c) may be seen to right and left of center. 



116 



Fig. 120.—Crater and cracks formed by lamped explosion of 8.4 pounds of TXT, 4 feet 6 inches under 

Shelter B(d). 



Fig. 121.—Showing damage to the floor of Shelter B(d) by tamped explosion of 8.4 pounds of TNT, 4 feet 6 
inches under the shelter. The strain gage type of accelerometer is also shown. 








117 



Fig. 122.—Separation of earth behind Shelter B(d) from the vertical displacement of the shelter resulting 

from the tamped explosion. 



Fig. 123.—The vertical displacement of Shelter B(d) following tamped explosion of 8.4 pounds of TNT, 4 
feet 6 inches under the shelter may be observed in this photograph, taken with the shutter open during 
the explosion. The extent of the displacement is indicated by the shadowy double exposure above 
the shelter and by the dark line above the bent bar leaning against the rear of the shelter, 


41 - 


420r)04 


9 







118 


Tesl on Shelter August 21, 1941 

Instniments .—In addition to the acceieroniotor and the pressure 
"age, four inecbanieal displacement gages of telescoping type were 
used in this test. Fig. 124 shows a view of the instrument set-up. 
The hole in the floor slab for the pressure gage was located 2 feet 
from the inside face of the rear wall and 7 feet 2 inches from the east 
side wall. The accelerometer was placed at 2 feet 6 inches from the 
rear wall and 4 feet 4 inches from the east side wall; and the displace¬ 
ment gages were placed along a longitudinal line 2 feet 9 inches from 
the inside face of the rear wall. 

Explosive charge .—The 8.4-pound charge was placed at a depth of 
6 feet 1 inch, located longitudinally midway between the two ends of 
the shelter and vertically in a plane passing through the ground 
intersection line. 

Explosion damage.- As anticipated, the explosion resulted in a 
camouflet, approximately 9 feet deep, its center located about 5 feet 
3 inches below ground level, with a camouflet radius of approximately 
2 feet 3 inches. Physical damage to the shelter consisted of the fol¬ 
lowing: two typical longitudinal cracks in the inside face of the flooi- 
slab; two noncontinuous vertical cracks in the inside face of the rear 
wall, located near a construction joint at midspan; two vertical cracks 
in the outside face of the rear wall, located in the middle third of 
the span; and three noncontinuous vertical cracks in the inside face 
of the front wall. The four displacement gages were shattered and 
no readings were obtained. An accelerometer reading of 97 g was 
obtained and a maximum pressure gage reading of 995 ± pounds per 
square inch with a total pressure duration of 0.0076 second. The 
vertical displacement of the shelter as recorded by the motion pictures 
was 3.0 inches in front and 6.7 inches at the rear. Fig. 125 shows a 
view of the inside of the shelter after the explosion. Figs. 126 and 127, 
views under the front edge of the shelter before and after the explo¬ 
sion, and Fig. 128, a view of the opening excavated in the top of the 
camouflet. The trace of the vertical displacement of the shelter due 
to the explosion pressure wave is shown in Fig. 129 in the form of a 
shadowy double exposure above the roof outline. 

Test on Shelter A(a), August 28, 1941 

Instruments .—Instruments for this shelter were the same as those 
used in Shelter B(a). The hole in the floor slab for the pressure gage 
was located 6 feet 7 inches from the inside face of the rear wall and 
15 feet 8 inches from the east side wall. The accelerometer was 
placed 4 feet 1 inch from the inside face of the rear wall and 8 feet 
10 inches from the east side wall; and the four displacement gages 
were located along a longitudinal line 5 feet 10 inches from inside face 
of the rear wall, all as shown in Fig. 130. 


119 



Fir,. 124.—Iiisiiuinent.s used for the tamped exjilosion test on Shelter B(a). 1 shows the mechaideal 

disitlacement gattes and 2, the accelerometer. 



Fig. 125.—Showing damage to the instruments in Shelter B(a) bj- tamped e.xplosion of S.4 pounds of TNT ,(i 

feet I inch below the shelter. 



120 



Fui. 12ti 



Fig. 127.—Views under Shelter B(a) before and after tamped explosion of 8.4 pounds of TNT resulting in a 

camouflet. 





121 



Fig. 129.—Photograph with shutter open during the tamped explosion under Shelter B(a) showing the 
vertical displacement of the shelter by the double exposure above the roof line of the shelter. 





122 



Fir,. 130.— 1 )i>iilaceinent gages and accelerometer used in the tamped exiilosion test on Shelter A(a). 



Fig. 131.—Damage to instruments and Shelter A(a) by tainpe;] explosion of 05 pounds of TNT, 11 feet 

under t he slielter. 






123 


Explosive charge.—T\\e deptli of the 65-])oiind cliarge, measured to 
the upper shell of the container, was about 11 feet. It was located 
longitudinally midway between the two ends of the shelter and directly 
below the ground intersection line. 

Explosion damage. —As expected, the explosion resulted in a camou- 
fiet. While the cavity was not of regular form, being more like a shaft, 
its approximate center was located about 7 feet below grade and 6 feet 
inside the front edge of the shelter, and the radius of the camouflet 
varied from a minimum of 3 feet 6 inches, when measured in a hori¬ 
zontal plane, to a maximum of 8 feet when measured in the inclined 
direction of the shaft. Damage to the shelter was confined to typical 
cracking in the inside face of the bottom slab, including four longi¬ 
tudinal and radiating cracks, a single crack each in the inside faces 
of the rear and ceiling slabs, and extensive shattering of the loose 
concrete in the inside face of the front wall, which had been shattered 
by a previous explosion test. The displacement gages were again 
shattered. The accelerometer showed a reading of 140 g for 0.011 
second, and the pressure gage 990±pounds per sciuare inch with a 
pressure duration of 0.0056 second. The vertical displacement of the 
shelter from the motion pictures was 5.1 inches in front and 9.7 inches 
at the rear. Fig. 131 is an inside view of the shelter after the explo¬ 
sion, showing the debris of the stripped wall and parts of the broken 
displacement gages. Fig. 132 shows the earth heaved up under the 
shelter by the explosion; Fig. 133, cracks on the rear face of the 
shelter; and Fig. 134, the opening excavated in the top of the camouflet. 
Fig. 135 shows the extent of the vertical displacement by the double 
exposure above the roof outline. 

Tesl on Shelter A(b), September 4, 1941 

Instruments. —^The accelerometer was placed, as shown in figure 136, 
5 feet 8 inches from the inside face of the rear wall and 9 feet from the 
east side wall. The pressure gage was located directly above the 
charge and in contact with the underside of the bottom slab at a dis¬ 
tance 8 feet 8 inches from the rear wall. The displacement gage for 
this test consisted of a %-inch diameter round bar, connected to the 
top face of the bottom slab near the hole for the pressure gage, and 
extending vertically through the ceiling slab to form a gage mast of 
the floor displacement during explosion. 

Explosive charge. —The 64-pound charge was placed at a depth of 
12 feet below grade, measured to the top of the container, midway 
between two ends of the shelter and in a vertical plane passing through 
the gi*ound intersection line. The backfill was deposited in the trench 
in the usual manner. 

Explosion damage. —Contrary to expectation, the explosion did 
not form a camouflet, but a crater with a small opening at the top. 
The shape of the crater was that of an inclined shaft, its deepest 


124 



Fig. 132.—Showing the upheaval of earth under Shelter A(a) following tamped explo.sion of 65 iiounds 

of TNT, 11 feet under the shelter. 



Fig. 133.—Cracks formed on the rear of Shelter A(a) may be seen at the center and left of center. 













IB 


125 




Fig. 134.— Opening e.xcavated in the top of the camouflet under Shelter A(a). 


Fjc;. 135.—Photograph with shutter open during the tamped explosion under Shelter A(a), showing the 
vertical displacement of the shelter by the double exposure above the roof line of the shelter. 


126 



Fig. 137.—Interior of Shelter A(b) following tamped explosion of 64 pounds of TNT, 12 feet under the 

shelter. 





127 


point at the inner end being about 12 feet 8 inches below grade and 
the inaxinunn transverse diameter 8 feet 8 indies. Damage to the 
shelter was confined to minor hair cracks in the rear wall and floor 
slab and a vertical crack in the front face extending from the bottom 
edge to the center of the old explosion crater. The displacement bar 
gage showed on one frame of the motion picture record, a vertical 
displacement of 1.9 inches with respect to the shelter, itself in motion. 
The vertical displacement of the shelter itself was 7.6 inches. The 
accelerometer showed a reading of 25 g for 0.010 second, and the 
pressure gage, 995 ± pounds per square inch, with a pressure duration 
of 0.0091 second. As in the case of the other shelters, the explosion 
caused a bodily uplift followed by a forward drop, as indicated by a 
5-inch wedge opening between reai* wall and the backfill. Fig. 137 
shows the interior of the shelter after the explosion and Fig. 138 the 
crater formed by the explosion. Fig. 139 shows the vertical dis¬ 
placement of the shelter. 

Test oil Shelter B(h), September 13, 1941 

Instruments .— In addition to the accelerometer, piezo-electric gage 
and three telescoping type displacement gages, as i)reviously described, 
two Madugno copper-disk pressure gages and an additional acceler¬ 
ometer of the new eddy-current type, as illustrated in Fig. Ill, 
mounted on the base of the U-bar accelerometer, were used in this 
test. In the Madugno pressure gage, pressure is measured by de¬ 
formation of a copper diaphragm, calibrated by the application of 
hydrostatic pressure. In the eddy-curi-ent accelerometer, voltage 
proportional to acceleration is produced in electromagnetic pick-up 
coils by eddy currents m an aluminum vane accelerated in a magnetic 
field. The piezo-electric pressure gage was located 2 feet from the 
inside face of the inside wall and 7 feet 9 inches from the east side 
wall, while the two Madugno pressure gages were located 12 inches 
farther away from the rear wall. The accelerometers were placed 

2 feet 2 inches from the inside face of the rear wall and 4 feet 6 inches 
from the east side wall; and the three displacement gages were located 
on a transverse line 7 feet 10 inches from the east side wall and spaced, 
respectively, 1 foot 8 inches, 3 feet 6 inches and 5 feet 1 inch from 
the inside face of the rear wall. Figure 140 shows the arrange¬ 
ment of the instruments inside the shelter before and after the 
explosion. 

ET'plosive charge .—The 8.4-pound charge was placed about 6 feet 

3 inches below grade, at the center of a vertical plane passing through 
the ground intersection line. The dry earth fill was placed in the 
trench in layers and tamped in the usual manner. 

Explosion damage .—The explosion, which was of low order, caused 
no damage or any visible displacements, and, unlike all previous tests, 
no crater or camouflet of any shape was formed in the ground. A 


128 



Fig. 138.—Opening of the crater under Shelter A(b). 



Fig. 139.—Photograph with shutter open during the tamped explosion under SheltiT A(b) showing the 
vertical displacement of the shelter by the double e.\|)osure above the roof line of the shelter. 




129 



Fig. 140.-Interior of Shelter B(b) following tamped explosion of 8.4 pounds of TNT, G feet 3 inches under 
the Shelter. The detonation was low order with negligible effects. 



Fig. 141.—Minor upheaval of earth under Shelter B(b) by low order detonation of 8.4 pounds of TNT, 6 

feet 3 inches under the Shelter. 







130 


wisp of smoke and a slight ground shock were the only evidences that 
a detonation had occurred. No instrument readings were obtained. 
The vertical displacement of the shelter was 0.8 inch. Fig. 141 
shows a minor upheaval of earth under the shelter as a result of the 
low order detonation. 

Test on Shelter A(d), September 13, 1941 

Instruments .—The instruments were the same as those used in the 
preceding test. The accelerometers were placed 4 feet 5 inches from 
the inside face of the rear wall and 9 feet from the east side 
wall; the pressure gages were located 6 feet 6 inches from the inside 
face of the rear wall and 15 feet 6 inches from the east side wall; and 
the three displacement gages were spaced along a longitudinal line 
5 feet 1 inch from the inside face of the rear wall, the middle one being 
15 feet 6 inches away from the east side wall and the other two 1 foot 
9 inches and 1 foot 5 inches apart. Fig. 142 shows the arrange¬ 
ment of the instruments inside the shelter before the explosion. 

Explosive charge .—The 66-pound charge was placed 8 feet 1 
inch below grade, measured to the center of the container, at the 
center of a vertical plane passing through the ground intersection line 
of the bottom slab of the shelter. The dry earth fill was placed in the 
charge trench in layers and thoroughly tamped. 

Explosion damage .—The explosion resulted in a violent shock and 
severe damage to the shelter. Viewed from inside, a 2-foot strip 
of the bottom slab was ruptured longitudinall}^ at the junction with 
the front wall, as well as cracked transversely at the center causing 
an upward tilt. At the underside of the slab, directly above the 
charge, a crater was formed in the concrete measuring 7 inches deep 
and 36 inches in diameter. The rear wall, viewed from inside, had 
a vertical crack inch wide, located at midspan and extending from 
the bottom to midheight, and one large diagonal crack at each lower 
corner. The displacement gages were again shattered. The strain 
gage accelerometer gave a reading of 75 g for 0.0067 second. The 
eddy-current accelerometer showed 100 g. The piezo-electric pres¬ 
sure gage gave a pressure reading of 4,000 pounds per square inch, 
which after 0.0003 second dropped to 2,300 pounds per square inch 
with a total pressure duration of 0.003 second. The Madugno 
pressure gages showed a maximum pressure of 2,400 pounds per 
square inch. The vertical displacement of the shelter, as recorded 
on the motion pictures, was 8.1 inches in front and 6.6 inches at the 
rear. Fig. 143 shows an interior view of the shelter after the ex¬ 
plosion. The extent of cracking in the front wall is shown in Fig. 
144. The inclined crater shaft in the ground was about 9 feet 
deep at the inner end with a maximum transverse diameter of 
about 9 feet. Fig. 145 shows the cleavage between the earth behind 
the shelter and the rear wall. 


131 




Fig. 143.—Damage to iustrumeuts and Shelter A(d) by tamped explosion of 66.0 pounds of TNT, 8 feet 

1 inch under the Shelter. 






132 



Fig. 144.—Crater formed under Shelter a( d) by tamped explosion of OOinmndsof TNT, 8 feet 1 inch under 

the shelter. 



Fig. 145.—Separation of earth behind Shelter A(d) from the vertical displacement of the shelter result¬ 
ing from the tamped explosion. 








133 




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41 - 10 


















































































PART IV— DISCUSSION OF TEST RESULTS 

IMPACT PENETRATION EFFECTS 
Nature and Shape of Craters and Holes 

The nature of the craters and holes produced by impact penetration 
can best be appreciated by examination of the photographs of pene¬ 
tration effects and the contour sketches, Figs. 21 to 65, 79 to 86, 
and 96 to 107, inclusive. Impact penetration effects were localized 
and damage confined to areas adjacent to the point of impact. 

Diameter of Craters and Holes 

The maximum diameter of impact craters was approximately six 
times that of the projectile. The diameter of holes where perforation 
occurred, was roughtly one and one-half times that of the projectile. 

Scabbing 

The scabbing craters formed by perforation of the upper slabs of 
shelters A(b) and B(b) were 2.5 to 3 times larger than the spalling 
craters on the front faces, and were longer in the direction of the 
transverse steel which was next to the surface of the slabs. 

Reinforcing 

The reinforcing bars on the outer face of the slabs w(‘re bent out¬ 
ward by the impact and ejected detritus, and those on the rear fac(' 
were similarly bent outward by the rejected material in the cases 
where scabbing or perforation occurred. In only a few cases were' 
bars cut or broken by impact penetration. The manner in which 
the reinforcing bars were bent outward on each face suggested the 
desirability of attaching the transverse and longitudinal reinforcing 
at the faces securely to the shear reinforcing. 

Effect of Oblique Impact 

The tendency of a projectile which strikes obliquely, to deflect and 
pursue a curved path thi'ough the target material, which phenomenon 
is most apparent in a ricochet impact, was evident in all impact 
penetration tests, including those on the additional slabs. The 
deflection was most pronounced in an upward direction, as might be 
(‘xpected from the angle of impact. The lateral deflection was in some 
cases to the right, in others to the left. Figs. 146 and 147 show the 
path of the projectiles througb shelters A(b) and B(b). It will be 

( 134 ) 


135 



A(b) 



/ * 

/ 

/• 

/ 




/• 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 

/ 
/ 
/ 
/ 
/ 
/ 
/ 
/ . 




/ \ 


\ 


■At 


i” 

40 ^ 



A(d) 


■A- 


Fig. 146.—Vertical sections through impact penetration craters of the ?^-scalo shelters, showing the de¬ 
flection of the 6-inch projectile in perforating the ui)per slab of Shelter A(b). 






































136 


FLIGHT OF 
PROJECTILE 



B(b) 




B(a) B(c) 



B(cl) 


Fig. 147.—Vertical sections through impact penetration craters of the Me-scale shelters, showing the de¬ 
flection of the 3-inch projectile in perforating the upper slab of Shelter B(b). 































137 


noted that the pta-forations in tin* upper slabs arc' similar. In (‘ach case. 
the hole is roughl}^ elliptical and the plane of the hole is tilted back. 
In shelter A(b), the top of the hole is 9)^ inches, and the bottom of 
the hole 5% inches from the outer face of the slab. In B(b) the top of 
the hole is 5 inches, and the bottom 3)^ inches from the outer face of 
the slab. The general effect of oblique impact is to reduce penetration, 
first by reason of the longer diagonal path, and second, bc'cause of the 
further tendency of the projectile to deflect. 

Adjustment of Penetration Values 

The' penetration tests on the preliminary test slab, on test shelters 
A(c), A(d), B(c), and B(d), and to a lesser extent on test sheltcu's 
A(b), and B(b), furnish useful data on penetration for the determina¬ 
tion of the penetration coefficient for reinforced concrete, in the 
formula S=kPV'\ To permit comparison of the penetration results 
at the two scales and to determine from these results the value of the 
impact penetration coefficient for the reinforced concrete used in the 
test shelters, it was necessary to adjust the observed penetration read¬ 
ings to a common basis of striking velocity, obliquity and sectional 
jjressure. 

Striking velocity 

The penetration readings at the various striking velocities wen' 
adjusted to values at the nominal test velocity adopted of 1,000 

feet per second. The function F"=Logio ^ 1 + 915 qqq ^ ^ formula 

S=kPV'\ was used in making the adjustments. The adjusted values 
are given in column ( 2 ) of Table 21 . 


Ohliqi'ity 

As stated, pc'netration at oblique impact is less than for impact 
normal to the surface. Excluding a ricochet impact, the equivalent 
normal penetration of an oblique impact may be determined in one 
of two w’ays: first, using the measured normal penetration, by comput¬ 
ing the penetration along the trajectory of the projectile, assumed 
from the given obliquity; second, by computing the normal component 
of the impact velocity which corresponds to the measured normal pene¬ 
tration, and adjusting the penetration value to the observed velocity, 
or, in our case, to the arbitrary striking velocity of 1,000 feet per second 
used for comparing residts. 



138 


7’able 21.- Adjvsted penetration values 


Slab thickness 
(inches) 

Striking 
velocity 
(f. s.) 

Obliq¬ 

uity 

Test Shelter 

Penetration in inches 

d 

V 

e 


(1) 

(2) 

(3) 

(3a) 

(4) 

(4a) 

40^^_ 

981 

0 7~ 

21 00 

A(d)_ 

11. 75 

11. 97 

12.82 

12.80 

12. 82 

12.80 

31)^ 

1,023 

21 30 

A(c) 

14.50 

14.19 

15. 25 

15. 20 

15. 25 

15.20 

201.4 _ . 

979 

21 15 

B(d) 

6. 25 

6.38 

6. 84 

6. 83 

6.62 

6.62 

1554 _ 

1,014 

21 00 

B(c) - _ 

7. 63 

7. 52 

8.06 

8.04 

7.80 

7.78 

mi _ 

902 

18 00 

Test slab-__. 

6.50 

7. 19 

7.56 

7.57 

7. 32 

7.33 


(1) Observed penetration values. 

(2) Values (1) adjusted to 1,000 feet per second. 

(3) Values (2) normalized by the first method. ((2)/cos d). 

(3a)Values (2) normalized by the second method. 

(4) Values (3) of B models adjusted to A models for sectional pressure, 

\2 X 265y/ 

(4a)Values (3a) of B models adjusted to A models for sectional pressure, 

V2 X 265y/ 


8 ^ 0 =normal penetration due to oblique impact and striking- 
velocity Vo, 

normal penetration due to normal impact and Vq, 

0 = angle of obliquity of projectile impact, 
striking velocity of oblique impact, 

F„=normal component of T"o,= T'oCos 

Fo" = velocity factor for T"„ = Logio (^ + 915 pOo )' 

F„" = velocity factor for 1",^; 
then, by the first method, 

o _ So , 

Cosr 

and, by the second method, 

V " 

O _ O * 0 

^ n 

Similarly, if T 'h.ooo indicates the velocit}^ factor for a striking velocity 
of 1,000 feet per second, then the corresponding normal penetration, 
obtained from So by the two methods, is 
by the first method, 

and, by the second method, 

‘ n 

The test penetration values, adjusted for normal impact and a 
striking velocity of 1,000 feet per second, are given in columns (3) 
and (3a) of Table 21. 



































139 


Sectional pressure 

Through inadvertence, the weights and sectional pressures of the 
two sizes of projectiles used against the shelters of series A and B, 
respectively, were not exactly adjusted to correspond to the 2 to 1 
scale ratio of the test shelters, the sectional pressure of the 13-pound 
3-inch projectile being 265 pounds per square foot and that of the 
6-inch projectile being 513 pounds per square foot. For exact cor¬ 
respondence, the weight of the 6-inch projectile should have been 
104 pounds in lieu of 100.75 pounds, and the sectional pressure 
would then have been 530 pounds per square foot. To permit com¬ 
parison of the penetration results on the %-scale shelters with those 
on the Ke-scale shelters, it was necessary to adjust the penetration 
readings on the B or Ke-scale shelters to a projectile sectional pressure 
exactly one-half of that used on the % model. The penetration values 
adjusted for corresponding sectional pressure are given in columns 
(4) and (4a) of Table 21. 


Concrete strength 

While it is recognized that depth of penetration is a function of the 
compressive strength of concrete and that adjustment should, ac¬ 
cordingly, have been made for this factor, none was, nevertheless, 
made for the reason that neither the relation nor the actual concrete 
strengths were known. The test cylinder strengths, which were the 
only available criteria of concrete strengths, are very likely not 
representative of the increased 28-day strengths throughout the 
slabs at the time the penetration tests were made. They were prob¬ 
ably more nearly uniform than would appear from the cylinder 
strengths in Table 8. In ‘‘Civil Protection,’’ it is stated ^ that the 
penetration coefficients vary inversely as the compressive strengths 
of the concrete, but experimental observations of Skramtajew,^ which 
are supported by recent British tests, indicate that penetration values 
are affected by variations in concrete strength to a much less extent 
than stated in “Civil Protection.” Had the adjustment been made 
in accordance with the observations of Skramtajew, it would have 
favored the correlation of results and would have brought the pene¬ 
tration value on shelter B(d) into almost exact agreement with that 
on shelter A(d), and, similarly, B(c) with A(c). For the assumed 
probable 28-day compressive strength of 5,000 pounds per square 
inch for shelters A(d) and A(c), and 4,500 pounds per square inch 
for shelters B(d) and B(c), the corresponding relative depth of pene¬ 
tration, according to these observations, would be respectively 0.595 
and 0.625, or a variation of 5 percent. As the adjusted penetration 

• The source of the data is not given. 

2 Bazant, “The Use of Reinforced Concrete for Fortifications and Air Raid Shelters,” Travaux, April 
and May 1937, 



140 


values in coluinns (4) and (4a) of table 21, taking the sectional pres¬ 
sures into consideration, show a variation between B(d) and A(d) 
of 3.4 percent, and between B(c) and A(c) of 2.3 percent, the correction 
woidd, as stated, have brought these adjusted penetration values 
into almost exact agreement. However, in view of the lack of exact 
information, refinement in adjustment was not warranted. Since 
results were almost in agreement, it was considered that an average 
of the two vv^ould give a representative penetration coefficient for a 
concrete of a compressive strength in the range of 4,500 to 5,000 
pounds per square inch. 

Penetration Coellieient for Reinforced Concrete 

Using the penetration formula S=kPV" and the penetration values 
for test shelters A(d) and B(d), adjusted for normal impact at 1,000 
feet per second, the values of k are found to be respectively 0.00277 
and 0.00286, or a round value of 0.0028 for a class E reinforced con¬ 
crete of a nominal 28-day compressive strength of 3,000 pounds per 
square inch and an actual compressive strength of 4,500 to 5,000 
pounds per square inch. Had the penetration tests on the Prelim¬ 
inary Test Slab and shelters A(c) and B(c) also been used to determine 
the value of the constant k, different and higher values would have 
resulted. Such calculations were not made or used for reasons which 
wdll be discussed hereinafter. 

(lomparison M'ith British values 

Comparison of the penetration coefficient determined from the 
tests for the reinforced concrete in the shelters with the British 
results tabulated in ‘‘Civil Protection’' ^ indicates a reasonable agree¬ 
ment. For a reinforced concrete having a compressive strength of 
5,200 pounds per square inch and 1.4 percent of reinforcement, the 
penetration coefficient given in “Civil Protection” is 0.00308; for a 
5,700-pound concrete and the same percentage of reinforcement, k is 
given as 0.00282, compared with the k value determined by the shelter 
tests of 0.0028. The concrete used in the shelters was thought from 
the cylinder tests to have a 28-day compressive strength of 4,500 to 
5,000 pounds per square inch. In “Civil Protection,” however, after 
the impact penetration value is doubled to determine the limit or 
proof thickness, it is recommended that this theoretical thickness re¬ 
quired, to resist impact and explosion, be arbitrarily increased by a 
considerable amount averaging 58 percent for a 2,000-pound heavy 
case bomb. For example, the theoretical proof thickness of 5,700- 
pound concrete with 1.4 percent of reinforcement to resist the impact 


’ The source of the data in “Civil Protection” is not stated. 



141 


and explosion of a 2,000-ponjid bomb with a 30 percent charge- 
weight ratio, a sectional pressure of 1,900 pounds per square foot and 
a velocity factor V" of 0.947 corresponding to a striking velocity of 
1,300 feet per second, is given as 10.14 feet, but it is recommended 
that a thickness of 16.4 feet be used. While some increase would, no 
doubt, be prudent if a single slab were used, in view of the almost 
exact agreement between actual and predicted test results on both 
the shelters and additional slabs, there appears to be no reason for 
such conservatism when the preferred double slab type of roof con¬ 
struction is used, in which the second slab provides a secondary or 
reserve protection. It will be noted, however, that with the addition 
of the second or lower slab, the total thickness of roof protection pro¬ 
vided approaches the recommended British values. With reference 
to the tabular values of penetration given in ‘‘Civil Protection,’’ it 
may be remarked in passing that the sectional pressure of 1,900 
pounds per square foot given for a 2,000-pound bomb having a 
charge-weight ratio of 30 percent is entirely too high. By reference 
to Table I, it will be noted that a sectional pressure of such magnitude 
in this size of bomb would coiTespond either to a much lower ex¬ 
plosive content, approximately 10 percent, or to a considerably 
larger size bomb of the charge-weight ratio of 30 percent given. 

Effect of Slab Thickness on Penetration 

Graphic comparison of the adjusted i)enetration values on the Pre¬ 
liminary Test Slab and test shelters A(c), A(d),B(c), and B(d),with 
the coiTesponding slab thicknesses, discloses an interesting and im¬ 
portant phenomenon. See Fig. 148. It reveals that resistance to 
penetration in reinforced concrete, and undoubtedly in other elastic 
friable materials, is a constant only for slabs at least 3.25 times as 
thick as the depth of penetration ^ and that in the range of slab thick¬ 
nesses from 3.25 to 2 times the depth of penetration,^ the resistance 
offered to penetration is no longer a constant but isgradualh^-educed, 
and the penetration increased until perforation occurs in a slab ap¬ 
proximately twice such expected penetration depth. This may, 
perhaps, be more clearly explained by reference to Fig. 150. It will 
be noted, if the penetration in a slab of great thickness is taken as 
unity, that when the slab thickness becomes less than 3.25 times such 
penetration depth, the penetration gradually increases until perfora¬ 
tion occurs when the slab thickness is reduced to 2 times the pene¬ 
tration depth.^ This increased penetration results solely from the 
r(‘duction in slab thickness and without change in the projectile or 


1 In a slab of eroat thickness. 



Thickness Ratio ? Penetration in inches 


142 



Depth of Slab in Inches 

148.—Comparison of adjusted penetration values with corresponding slab thicknesses, showing the 
effect of relative slab thickness on the dei)th of penetration. 



Fig. 149.—Relation of relative slab thickness to penetration 












































































































































































































































































































































































































































































































































































Relative slabthickness 


143 



SLAB THICKNESS 

Fig. 150.—Pictorial representation of the effect of relative slab thickness on penetration. When the slab 
thickness is reduced, penetration increases without change in the projectile or striking velocity. 






















































144 


the striking velocity. The relation is shown graphically in Fig. 149, 
and for those who prefer formulae, is expressed in the form 

'p=l-{-e 
in which 

t= i] and 

Oi 

= penetration in slab of infinite tlnckness, 

5'= penetration in slab of d thickness, 

(Z—thickness of penetrated slab, 
e = base of natural logarithms (2.71828). 

Published information gives an incorrect impression of the action 
which takes place in penetration of a concrete slab short of perfora¬ 
tion. In ‘‘Civil Protection” it is stated: “The depth of penetration 
computed by the formula (revised Petry) is approximately correct, 
provided that the slab on which the bomb strikes is of a thickness 
equal to at least twice the penetration depth”; also, “The penetration 
of a bomb of fixed weight and velocity does not vary considerably in 
slabs of different thicknesses, provided that the thickness is always 
at least twice the penetration.” Fig. 150, and analysis of the test 
results show, on the contrary, that there is a considerable difference 
of penetration in slabs of different thicknesses as penetration ap¬ 
proaches perforation, and that penetration results are not consistent, 
unless the slab thickness is at least 3.25 times the penetration depth.^ 
It will thus be evident that scabbing does not occur suddenly, as 
might be inferred, but that internal scabbing or rupture begins, even 
though there may be no external visual evidence of same, when the 
slab thickness falls below 3.25 times the calculated penetration depth. 

This phenomenon explains much that has been irreconcilable in 
earlier published penetration tests when applied to concrete and 
similar clastic friable materials which have low tensile strengths as 
compared with the compressive, and has a bearing on the general 
theory of penetration, because, heretofore, it has been assumed that 
“the relative values of the resistance offered to bombs of various 
velocities V are the same for all penetration depths.” We now find 
that for concrete in the range of relative slab thicknesses mentioned, 
the depth of penetration is also a function of the relative slab thick¬ 
ness. What makes the discovery of particular interest, is that this 
range, in which the irregular behavior occurs, is likewise the range in 
which much of the experimental work has no doubt unwittingly been 
done. The foregoing now explains why only the penetration results 
on shelters A(d) and B(d) were used in the determination of the pene- 


> Tn a slab of great thickness. 



145 


tration constant for the concrete, and not the values on shelters A(c) 
and B(c), or of the Preliminary Test Slab, which because of the smaller 
ratio of slab thickness to penetration, would have given higher and 
incorrect results. It may explain the lack of correlation between 
the penetration constants determined by various earlier investigators, 
because these tests were in many cases, no doubt, carried on in the 
range of slab thicknesses in which, for the reason described, pene¬ 
tration constants can not be reliably determined, unless the effect of 
relative slab thickness is taken into account. The variation in pub¬ 
lished results is too wide to be ascribed solely to differences in the 
physical properties of the materials as has sometimes been suggested. 
The significance of this discovery, for future tests to determine pene¬ 
tration coefhcients of concrete and similar elastic friable materials 
subject to scabbing, is that test slabs should be 3.25 or preferably 3.5 
times the anticipated penetration depth in order that this phenomenon 
of what may be termed internal scabbing or rupture will not influence 
and confuse the results. 

Effect of Special Reinforcing 

The effect upon penetration of the special reinforcing used in the 
additional test slabs may be noted by referring to the photographs of 
penetration results, Figs. 79 to 86 and 96 to 107, inclusive, to the 
summaries in Tables 17 and 19, and to the comparison of theoretical 
limit velocities with the actual velocities and results tabulated in 
Table 22. The limit velocities given were calculated from the pene¬ 
tration formula S=kPV", using the penetration coefficient, 0.0028, 
the sectional pressure 280 pounds per square foot for the 3-inch, 13.75- 
pound projectile used and the stated obliquities. As previously 
explained, the limit slab thickness is twice the penetration depth 
calculated for the above conditions. Scabbing was not as severe or 
as extensive in the additional slabs with special reinforcing as it was 
in the shelters. The special reinforcing appeared to limit the damaged 
area and to retain the shattered concrete, in contrast to the tendency 
of the covermg concrete to strip off, noted in the shelters in which the 
conventional reinforcing was used. Impact craters were not reduced 
in area. There was some evidence in slabs D(al) and D(cl) that the 
depth of penetration was reduced. With an actual velocity of 698 
feet per second, as against a theoretical limit velocity of 687 feet per 
second, slab D(al) should just have been perforated but, actually, 
perforation was not quite complete. With an actual velocity of 991 
feet per second against a theoretical limit velocity of 1,020 feet per 
second, slab D(cl) should have been just short of perforation, but 
actually was short by an appreciable margin. These two tests un¬ 
supported by further evidence do not warrant the general conclusion 
that within usual percentages of steel used, the reinforcing can be so 


146 


disposed as to reduce impact penetration. In any case, however, 
from the overall etfects and reduction in scabbing, the desirability of 
using the special reinforcing in lieu of conventional reinforcing is 
indicated. The additional slab tests are of incidental interest in 
illustrating with what comparative accuracy, penetration depths, and 
required limit thicknesses of concrete to resist projectile and bomb 
penetration can be predicted when the important effect of relative 
slab thickness is taken into consideration. 


Effect of Aiitiscabbiiig Plates 


The impact penetration results on the 11%-inch slabs with plates, 
C(dl), D(bl), and D(b2), show that antiscabbing plates are effective 
in reducing perforation. With striking velocities of 968, 954, and 999 
feet per second, respectively, compared with a theoretical limit velocity 
of 840 feet per second, perforation of ll}^-inch slabs without plates 
would have occurred with a considerable margin, but, actually, whih' 
the concrete was completely penetrated, the steel plates were not 
perforated. Approximately equivalent resistance to perforation can 
be provided at half the cost of the steel plate, by an additional foot 
thickness of concrete, but the use of antiscabbing plates may be 
desirable under certain conditions. 

Table 22. — Theoretical limit velocities for Additional Test Slabs, showing the effect 
of special reinforcing and antiscahhing plates 



; Thiek- 
ness 


Striking velocity 


Slab 

Obliquity 

Actual 

Theoreti¬ 
cal limit 

1 Remarks 


Jnchea 

0 / 

/. «. 

/. s. 


C(al)- 

9 

19 00 

873 

088 

This slab should have been and was perforated 
with considerable margin. 

C(bl)- 

1 9 

19 00 

988 

088 

1 This slab should have been and was perforated 
even with a plate. 

C(cl)- 

1U4 

19 00 

977 

810 

Perforation occurred and should have. 

C(dl)_ 

> UH 

19 00 

908 

810 

The projectile completely penetrated the concrete 
but not the plate. A slab without a plate would 
have been easily perforated. 

D(al)- 

9 

18 30 

098 

087 

Perforation should have just occurred but did not 
quite occur. 

U(a2)... 


20 10 

080? 

093 

Xo velocity reading was obtained; result was 
inconclusive. 

n(bl)_ 

1 1114 

19 10 

954 

811 

A slab w’ithout a plate would have been easily 
perforated; actually perforation of the plate did 
not occur. 

D(b2)_ 

> UH 

18 30 

999 

838 

This slab likewise was not perforated but without 
the plate would readily have been perforated. 

D(cl).... 

13 H 

20 35 

991 

1,020 

This slab should not quite have been perforated; 
actually was not by an appreciable margin. 

r)(c2)_ 

13H 

19 10 

1.001 

1.010 I 

This slab should not and actually was not quite, 
perforated. 


' With \i'' stocl plato attached to the rear face. 


Scale Effect—Comparison of Models and Prototypes 

Comparison of the impact penetration values on the A series of 
shelters at % scale with those of the B series at Yu scale, indicates a 



























147 


close correlation of results with no significant scale effect in respect to 
penetration. The additional tests on slabs furnished further evidence 
to this effect. The direct scale relation, which the sizes and depths of 
craters and over-all damage on the models at one scale bear to those 
at the second scale, indicates, when proper consideration is taken of 
the effect upon penetration of the relative slab thickness, that the 
test results and findings on the reduced scale models may be extended 
to the full size prototype structures with the expectation that the 
behavior of the latter will follow predictions well within the range 
of accuracy practicable for field construction conditions. 

EXPLOSION EFFECTS 
Nature of Craters 

The general effect and the nature of the craters and scabbing pro¬ 
duced by explosions on reinforced concrete are shown in the photo¬ 
graphs. The shattering effect on concrete edges from nearby ex¬ 
plosions is evident in the photographs of explosion tests on the 
additional slabs. Figs. 88, 92, and 94. In general, explosion effects 
were more widespread than penetration effects. Explosions in the 
incomplete projectile penetrations enlarged and deepened the craters, 
though by no means in a uniform manner or degree. The worst 
damage occurred in shelter A (a) in which the TNT charge had end 
contact with the inner roof and rested in a 10-inch diameter hole 
in the muddy earth between roofs. This detonation almost literally 
exploded the two slabs, practically wrecking a large area of the roof of 
the shelter. It illustrates the aggravated effects of a tamped explosion 
in wet soil. The detonation of the first TNT charge in the corre¬ 
sponding Ke-scale shelter B(a), placed in slightly damp, loose sand 
about midway between roofs resulted in neglibible damage to either 
roof. No scabbing occurred. In order to make the test more nearly 
complete and comparable, a second 2.8-pound TNT charge was in¬ 
serted through a pipe and placed with the end in contact with the 
inner roof. The pipe was withdrawn, sand covered the charge, and 
the charge apparently detonated high order, but still gave no important 
damage to either roof. The results, as compared with those on shelter 
A (a), are anomalous, unless a large scale effect is assumed in this 
particular conditioning. It is possible that a difference existed in 
quality of concrete or that some important difference developed in 
the action of the detonation in a mud hole as compared with that in 
fairh" loose sand. In contrast with the irregular effects of explosions 
in incomplete penetration craters, much more uniform effects resulted 
from the charges detonated on the surfaces of shelters A(c), A(d), 
B(c), and B(d) and of slabs C(a2), C(b2), C(c2), and C(d2). The 
diameters and depths of craters, the scabbing areas on the rear faces 
and the limit or proof thicknesses were reasonably comparable, as may 


148 


be seen from the photographs and the summaries of results, Tables 
16 and 18. On comparing the two scales, all these dimensions appeared 
to vary approximately as the cube root of the amoimt of explosive 
used, or, therefore, directly as the linear scale. 

Scabbing 

Serious scabbing occurred on the ceiling of shelter A (a), and also 
on the ceilings of shelters A(b) and B(b) in which the TNT charges 
were detonated in the penetration craters in end contact with the inner 
roof slabs. Scabbing was relatively less in shelter B(b) than in A(b). 
The scabbing area on the ceiling of shelters was longer in the direction 
of the transverse reinforcing which was nearest the surface. 

Reinforcing 

While the supplementary explosions on shelters A(c), A(d), B(c), 
and B(d) exposed and, in some cases, broke the reinforcing steel, 
there was no apparent tendency to destroy the bond between con¬ 
crete and steel, except in the immediate vicinity of the explosion. 

Effect of Slab Thickness on Explosion Penetration 

Though the quantitative effect is not as clearly defined as in the 
case of penetration effects, there is evidence of the influence of relative 
slab thickness on the limit or proof thickness required to resist ex¬ 
plosion penetration. This factor must be taken into consideration 
in attempting to calibrate a penetration formula in terms of weight 
of explosive, although additional tests are required to establish the 
relation and the material constants. 

Calibration of Explosion Penetration Formula 

The only explosion test result which was suitable for use in cali¬ 
brating an explosion penetration formula was the one on test slab 
C(c2) in which the detonation of 8.4 pounds of TNT did not quite 
perforate the slab, as may be observed in Fig. 93. The slab thick¬ 
ness of 11)^ inches was accordingly taken as the proof thickness for 
this amount of explosive, and the value of the explosion penetration 
coefficient c' in the formula *S'(feet) =2c'VC'(pounds) determined there¬ 
from. The determined c' value of 0.23 for the class of concrete 
used in the shelters and test slabs may be compared with values given 
in “Civil Protection’’ of 0.20 for a 4,700- to 5,200-pound concrete which 
probably corresponds to the concrete used in the shelters and slabs, 
and a value of 0.22 for a 3,700- to 4,200-pound concrete. The source 
of the data in “Civil Protection” is not given. 



149 


Comparison of Theoretical Limit Thicknesses with Test Results 

It will also be of interest to compare the theoretical limit thicknesses 
of concrete with the thicknesses which remained in the shelter slabs 
following impact penetration and to review the behavior of the latter 
in the subsequent explosion tests. Using the explosion penetration 
coef ficient c'= 0.23 as determined above in the formula AS'(feet)= 
2c -\/f^(pounds) for the 4,500 -to 5,000-pound concrete used in the 
shelters, the limit thicknesses of slabs which would just be perforated 
by amounts of explosive used in the regular tests on the two sizes of 
shelters, are as follows: 

B shelters (2.8 pounds TNT), 7% inches, 

A shelters (22.2 pounds TNT), 15K inches; 

and in the supplementary tests, simulating explosions of bombs with 
66 percent explosive filler, 

B(c) and B(d) shelters (8.4 pounds TNT), 11)^ inches, 

A(c) and A(d) shelters (66.6 pounds TNT), 22)^ inches. 

Shelter A{h ).—The inner slab thickness of 22K inches,less the impact 
penetration depth of 2% inches, left a net thickness of 19% inches to 
resist the explosion effects of 22.2 pounds of TNT detonated in the 
crater. Since the limit thickness, according to the forumla, is 15% 
inches, perforation was not expected. Wliile penetration was not 
complete, being 20% inches as compared with a slab thickness of 
22% inches, when considered in conjunction with the scabbing inside, 
the result was the equivalent of perforation. 

Shelter Aic ).-—^The slab thickness of 31% inches, less the impact 
penetration depth of 14% inches, left a net thickness of 17 inches to 
resist explosion effects. With a limit thickness, as stated of 15% 
inches, perforation was not expected, but actually almost did occur, 
the reinforcing steel on the rear face being exposed in the bottom of 
the crater. 

Shelter B{h ).—The inner slab thickness of 11% inches, less the impact 
penetration depth of 1% inches, left a net thickness of 9% inches. The 
limit thickness according to the formula is 7% inches. Perforation 
was, accordingly, not expected, and did not occur. 

Shelter B{c ).—The slab thickness of 15% inches, less the impact 
penetration depth of 7% inches, left a net thickness of 8% inches. 
With a limit thickness of 7% inches, perforation was not expected and 
did not occur. 

Preliminary Test Slab .—The slab thickness of 16% inches, less the 
impact penetration depth of 6% inches, left a net thickness of 10 inches. 
Perforation did not occur though pronounced scabbing did. 


420504° 41-11 



150 


Supplementary explosion With a limit tliicknoss of 22}^ inches 

for the A shelters, as compared with actual of 31K hiclies and 40}2 
inches, and a limit thickness of \\){ inches for the B shelters, as com¬ 
pared with actual of 15% inches and 20% inches, neither perforation 
nor scabbing was expected, and did not occur in the (d) shelters and 
presumably not in the (c) shelters, although the rear faces were, of 
course, not available for inspection. 

Internal rupture or shattering of the concrete by prior impact and 
penetration, though not apparent, may have facilitated subsequent 
explosive penetration, particularly ui those slabs in which impact 
penetration was appreciable. 

Effect of Special Reinforcing 

Since scabbing did not occur on slab C(c2), only the result on test 
slab C(a2) was available to show the effect of the special reinforcing 
on explosion scabbing. Compared with the effect on the inner slab 
of shelter B(b), the special reinforcing appeared definitely to limit 
and reduce the scabbing. The scabbing area from the detonation of 
2.8 lbs. TNT on shelter slab B(b) of a residual thickness after impact 
penetration, of 9% inches, was approximately 32 inches wide by 40 
inches high, whereas the scabbing area from the detonation of three 
times this amount of explosive on test slab C(a2), which was 9 inches 
thick, was only 22 by 20 inches, and the scabbed material was re¬ 
tained over half the area. There was no tendency for the concrete to 
strip off unequally in the direction of the transverse reinforcing, as had 
been observed in the shelter. While the conditioning for these two 
tests was not identical in that the test slab was supported on a wooden 
frame filled with sand, whereas the shelter slab was imsupported 
beneath, and the explosion on the shelter slab occurred between the 
top and inner slab, it is not believed that either of these factors had 
any significant effect upon the relative results. 

Effect of Antiscabbing Plates 

From the explosion results on test slabs C(b2) and C(d2), it is evi¬ 
dent that antiscabbing plates prevent the scabbed material from 
being rejected, but they do not appear to reduce the damage to the 
slab. Their presence on the bottom side of slabs would make inspec¬ 
tion impossible and removal of damaged concrete and repahs to the 
slabs a tedious operation. 

Scale Effect—Comparison of Models and Prototypes 

There is some evidence of scale effect and greater relative damage to 
the %-scale models in the (a), (b) and (c) shelters, but not at all in the 
(d) shelters, in the (c) and (d) shelters from the supplementary ex- 


151 


plosion tests, or in the explosion tests on the additional slabs. While 
there was some evidence of scale effect as noted, it was not of suffi¬ 
cient extent as to require any large factor of safety in extending the 
results to the full scale prototype structures. 

INSTia MENTAL RESULTS 
Concussion Effects 

Concussion effects determined from gage readings, as distinguished 
from physical damage in the roof slabs, were relatively mild and of no 
great significance in the case of projectile impacts, were also not 
excessively severe from explosive charges which were not in actual 
contact with the ceiling slabs when detonated. However, when 
explosive charges were detonated in direct contact with the ceiling 
slabs, as in shelters A(d) and B(d), which have roofs of single solid 
slabs, and in shelters A(b) and B(b), in which detonation occurred 
against the ceiling slabs as a result of perforation of the upper slabs, 
concussion effects were violent and slab deflections large. Not only 
the slab deflections but also the initial velocities and accelerations of 
the slabs and the measured concrete strains were of a high order. 
The initial velocities of slabs of shelters A(d) and B(d), for example, 
were estimated at over 25 feet per second and the accelerations during 
0.01 second, at 100 g, as compared with an initial velocity of 3 feet 
per second and acceleration during 0.01 second of 15 g, in shelter 
A(c). Concussion effects from explosions were not as violent in 
shelters A(b) and B(b), as in shelters A(d) and B(d), presumably for 
the reason that explosions took place in the penetration craters which 
were above and not directly opposite the gages, as in the case of 
shelters A(d) and B(d). These results indicate the need of an air 
space to act as a cushioning medium between the upper slab, which 
receives the shock of impact and explosion, and the ceiling slab of the 
shelter, and correspondingly indicate the desirability of avoiding 
single solid slab roofs for power plants, communication centers and 
personnel shelters, in order to minimize damage to equipment, instru¬ 
ments, electric fixtures and piping attached to walls and ceilings, and 
to reduce the physiological and psychological effects of such violent 
concussion upon personnel inside. The need of making the upper 
slab of a double slab roof of sufficient thickness to prevent perforation 
is also indicated, in order that subsequent detonation may not occur 
against the ceiling slab. 

Increased Air Pressure Inside Shelters 

No evidence was obtained from any of the blast gages to justify 
the concern expressed in ‘‘Civil Protection” that even impact con¬ 
cussion of a 500-pound bomb might cause a pressure increase of the 


152 


air inside a shelter of as mnch as 15 pounds per square inch. Such a 
pressure increase, if realized, it is assumed would be harmful to person¬ 
nel. One lar^e reading of 8 pounds per square inch was registered 
during the explosion tests on the piezo-electric gage in shelter A(l)), 
but this gage was in the open doorway facing outward, as was the 
piezo gage in shelter A(a), which registered the second largest pres¬ 
sure increase of 1.9 pounds per square inch. Increased air pressure 
indications from blast gages near the ceiling inside the shelters were all 
of a low order. There appears, therefore, no need to be alarmed over 
the possibility of a harmful increase in air pressure inside the shelters 
from the impact or explosion of bombs on the roof. Nevertheless, 
the relatively large pressure readings in the doorways from explosions 
around the corner on the roofs suggests the need of adequate strength 
in shelter doors, and of indirect entrances so that doors will not be 
directly exposed to blast and splinter effects from bombs which strike 
nearby. The advisability of keeping shelter doors closed during 
bombing raids is also evident. 

Comparative Destruction by Demolition and Armor-Piercing 

Bombs 

It was assumed in adopting the basic bomb and the procedure of 
investigating penetration and explosion effects separately, that an AP 
type of bomb, because of penetration prior to explosion, would be 
more destructive than a demolition bomb of the same weight, not¬ 
withstanding the much greater explosive content of the latter. That 
this assumption was justified may be noted by referring to Table 23, 
which gives a comparison of the size and depth of craters formed by 
demolition bombs detonating on the surface, with those of AP bombs, 
containing one-third as much explosive, detonating after maximum 
penetration. 


Table 2S.--Comparative destruction by demolition and armor-piercinq bombs 


Shelter 

Slab 

thick 

ness 

Crater of demoli¬ 
tion bomb 

Explo¬ 

sive 

Crater of AP 
bomb 

Explo¬ 

sive 

Size 

Depth 

Size 

Depth 


Inches 

Inches 

Inches 

Pounds 

Inches 

Inches 

Pounds 

A(c) _ 

31H 

36x36H 


66.6 

65x74 

limit 

22.2 

A(d) _ 

40>2 

36x33 

6% 

66.6 

65x68 

1314 

22.2 

B(c) _ 

15M 

15x16 


8.4 

30x36 

9 

2.8 

B(d). ... _ 

20H 

1414x13^ 

3^6 

8.4 

30x46 

7^ 

2.8 


TAMPED EXPLOSION EFFECTS 

The character and extent of damage to the test shelters from tamped 
explosions under the shelter floors may be noted in the photographs. 
Figs. 114 to 145, inclusive. Concussion was violent and damage 





















153 


severe in the case of the lU'arer explosions, corresponding to the detona¬ 
tion of 1,300 pounds of TNT, 21 feet under the prototype shelter, 
appreciably less severe for the explosions farther away, corresponding 
to a depth of 32 feet in the prototype. Damage to the floor slab was 
general rather than localized, but such as might be caused by a heavy 
concentrated impact loading. As was expected, craters were formed 
by the nearer explosions in the earth under the shelter floors, and cam- 
ouflets from the deeper explosions, except under shelter A(b) in which 
the camouflet opened somewhat at the top, forming a crater, and also 
under shelter B(b), in which the explosion was of low order with neg¬ 
ligible effects. With this one exce])tion, all explosions were high order. 

Over-all Damage 

Unlike the localized impact and surface explosion effects of previous 
tests, damage to the floor slabs from the tamped explosions was some¬ 
what more distributed, similar to what might be expected from a con¬ 
centrated impact loading. Surprisingly enough, in view of the consid¬ 
erable thickness of earth between the bottom of the slabs and the cen¬ 
ters of the explosions, definite craters were formed in the concrete in 
the underside of two of the shelters, B(c) and A(d), indicating a more 
concentrated cone of earth pressure than would normally be expected. 
Damage to the slabs consisted of a typical cracking pattern in the 
floor, more pronounced in the center, continuous longitudinal cracks 
at the fillets between the floor and longitudinal walls, and transverse 
cracks in the front and rear walls centering near the construction 
joints. Figs. 115, 117, 121 and 143, which show the extent of the 
damage to the bottom slabs from the nearer explosions, indicate 
typical failures of rectangular slabs supported on all four sides. As 
might be expected, the main cracks running in the longitudinal direc¬ 
tion, remain parallel to the longitudinal supports in the middle half, 
indicative of large transverse stresses in that zone, then radiate to the 
corners at the ends where strains tend to equalize in both directions. 
In addition, the large and continuous cracks at the upper edges of the 
fillets between the bottom slab and longitudinal walls clearly illustrate 
tensile failure due to negative moments at the transverse supports, 
similar to that of a rigid frame bent under static loading. Th(‘ 
vertical or transverse cracks in the two side walls are indicative of 
excessive longitudinal strains. 

Instrumental Results 

The instrumentatimi for the tamped explosion tests was not altogether 
satisfactory. Some of it was developed during the course of the tests, 
with the residt that the experimentation and changes in instruments 
pr('V(‘nted verification of instrument readings in some cases. In otlu'r 
cases no readings were obtained. 


154 


Displacement 

Only one displacement reading of the shelter floor slabs was obtained 
with the telescopic gages, namely, in shelter B(a) where the average 
reading of the four gages was about 3 inches. The bar gage used in 
shelter A(b) showed a deflection of 1.9 inches of the floor, relative to 
the rest of the shelter, lasting 0.05 to 0.1 second. This represents a 
minimum value of displacement since the motion was observed by a 
motion picture record and showed on only one frame. The displace¬ 
ment therefore might have been greater between frames (24 per 
second). 

The displacement of the shelters as a whole was recorded on the 
motion picture records. These records show that the explosion im¬ 
parts an initial velocity to the shelter in a time less than 0.025 second, 
and the remainder of the motion is completed with only gravitational 
forces acting. See Fig. 151. The shelters rocked slightly forward 
and backward, contingent upon the location of the center of force 
relative to the center of gravity. The shelter displacement readings 
from the motion picture records were not satisfactorily consistent. 
If the soil pressures on the undersides of the shelters were assumed to be 
uniform for each of the two cases of nearer and farther explosions, the 
displacements of the B shelters should have been double those for the 
A shelters because of the relative weights and bottom areas; and the 
displacements for the nearer explosions should have been 2b times those 
for the farther explosions; that is, varying inversely as the distances 
squared, from the explosions. They did not vary in this manner as may 
be noted in the summary in Table 20. In view of the apparently con¬ 
sistent earth pressures, as evidenced by the gage readings and also by 
the crater and camouflet formation and over-all damage to the shelters, 
the apparent inconsistency in the shelter displacement readings cannot 
be explained unless we assume a longitudinal as well as transverse 
rotation of the shelters sufficient to affect the results, some defect in 
the technique of determining displacement by motion picture records, or 
errors in measurements or calculations. Notwithstanding the apparently 
inconsistent results, it is evident from both the motion pictures and still 
photographs taken with the camera shutter open, that a vertical dis¬ 
placement of several inches occurred in the models, and that even the 
full size prototype structure would be lifted bodily 2 or 3 inches, since 
the displacement would be three-eighths the indicated readings for the 
A shelters, or three-sixteenths those for the B shelters. 

Acceleration 

No floor slab acceleration readings were obtained on shelters B(c), 
A (c) or B (d). The readings on shelters B (a) and A (a) were of doubtful 
accuracy. Excessively high readings, 680 g and 960 g, respectively, 
were shown, indicating inaccurate calibration probably from changes 


155 



Fig. 151.—Displacement taken from motion-picture records of front and rear corners of Shelter B(d), from 
a tamped explosion of 8.4 pounds of TNT 4 feet 6 inches under the shelter floor. 




Fig. 152 .—Earth pressure records by the piezo-electric gages from tamped explosions of 8.4 pounds of TNT 
6 feet 1 inch under Shelter B(a) and 66 pounds of TNT 8 feet 1 inch under Shelter A(d). 



















































































































































































































































































































156 


in the instruments due to the shock of detonation. The gage was 
damaged by loose concrete which fell from the front wall of shelter 
A (a), necessitating replacement of the strain element, and preventing 
recalibration of the gage. The acceleration values in Table 20 for 
shelters B(a) and A (a) were computed assuming the same sensitivity 
as in later tests on A(b) and A(d) for which reproducible calibrations 
were obtained. On shelter A(d) a reading of 75 g was obtained on 
the strain gage accelerometer, and a reading of 100 g by the eddy- 
current accelerometer. 

The acceleration readings are obviously inconsistent. Assuming 
approximately the same ^ unit soil pressure on the undersides of shel¬ 
ters B(a), B(b), A(a) and A(b),and uniformly higher values for shelters 
B(c), B(d), A(c) and A(d), under which the nearer explosions oc¬ 
curred, the slab and shelter acceleration values should have varied 
inversely as the scale ratio of the models; that is, twice as great for 
the B models as for the A models. The acceleration values should 
also have varied inversely as the ratio of the distances squared from 
the slabs to the explosions; that is, approximately 2}^ times as great 
for the nearer explosions as for those farther away. Since they did 
not bear this relation one to the other, and in view of the difficulties 
which attended the taking of most of the readings, the acceleration 
values cannot be regarded as reliable, and should be used only for 
order of magnitude. It is believed that the floor slab accelerations 
may have approximated the following: 

B(a) B(b) A (a) A(b) B(c) B(d) A(c) A(d) 

80g 80g 40g 40g 180g 180g 90g 90g 

By observing the height of rise of shelter B(d) from the motion 
picture record. Fig. 151, calculating the initial velocity, and using 
the observed shape and duration of the pressure curve, the maximum 
acceleration of the B shelters from the nearer explosions was calcu¬ 
lated as about 25g. The corresponding acceleration for the A shelters 
would be 12.5 g and for the deeper explosions, llg and 5.5 g, re¬ 
spectively. That the forces involved are considerable will be appre¬ 
ciated when it is realized that shelters B(a) and B(c) weigh 75 tons; 
B(b), 66 tons; B(d), 60 tons; and shelters A (a) and A(c), 600 tons; 
A(b), 532 tons; and A(d), 480 tons, including 9 tons of sand for the B 
shelters and 70 tons for the A shelters. 

The corresponding slab and shelter accelerations for the full size 
prototype structure would be three-eights those of the A shelters, or 
respectively 15 g for the floor slab and 2 g for the entire shelter, in 
the case of explosions of 1,300 pounds of TNT 32 feet under the 

1 The. amounts and positions of explosive were, as stated, selected to produce identical unit soil pressures 
under the models of both scales for each of the two cases, and the consistently similar damage to the shelters 
indicated that the unit soil pressures were uniformly about the same for the two cases of nearer and farther 
explosions. 



157 


shelter, and 34 g for the floor slab and 4.5 g for the shelter, for the 
same explosions only 21 feet under the floor slabs. The former would 
at least throw personnel off their feet and machinery out of adjust¬ 
ment, and the latter would undoubtedly result in broken ankles and 
equipment. The magnitude of the explosive forces involved suggests 
the desirability of preventing the occurrence of such destructive explo¬ 
sions under the floor slabs, rather than to attempt to design the floor 
slabs to resist them. 

Earth pressures 

The explosion under the first shelter tested, shelter B(c), resulted 
in a pressure beyond the calibration range. A value between 2,000 
and 3,000 pounds per square inch was obtained by extrapolation. In 
the next two tests, the gages were farther away and not facing the 
charges. The readings were, accordingly, not of significance and 
were not recorded. The next three readings on shelters B(a), A(a), 
and A(b) were all satisfactorily consistent at slightly less than 1,000 
pounds per square inch. In the last test on shelter A(d), the piezo¬ 
electric gage reading reached a maximum of 4,000 pounds per square 
inch but after 0.0003 second dropped to 2,300 pounds per square inch. 
A check reading of 2,400 pounds per square inch was obtained on the 
Madugno gages in this same shelter. If we accept 1,000 pounds per 
square inch as a representative unit soil pressure for the shelters 
under which the explosive was farther away, the corresponding pres¬ 
sure for the nearer explosions would be 2,250 pounds per square inch, 
which is in reasonable agreement with the readings actually obtained. 
It should be realized that these maximum pressures are of very short 
duration, the total pressure interval being only 0.0056 second to 
0.0091 second for the farther explosions. The nature of the pressure 
curve is shown in Fig. 152. The second peak on the B(a) record 
may have been caused by upheaval of improperly packed earth. 

Comparison of Earth Pressures with Acceleration Values 

Some check between earth pressures and acceleration values may 
be obtained by calculating the unit earth pressure which would have 
produced the acceleration for shelter B(d) of 25g, and comparing this 
value with the estimated pressure reading of 2,250 pounds per square 
inch. Substituting in the expression “force equals mass times acceler¬ 
ation,’’ the acceleration value, 25 g, the weight of the shelter, 60 tons, 
and the area of the bottom slab in contact with the earth, 133.6 square 
feet, gives a value of 156 pounds per square inch, representing the 
average unit pressure, which, if exerted during the pressure interval 
uniformly over the entire contact area of the slab, would have pro¬ 
duced the stated acceleration of 25g. The maximum pressure would 
be double the above average value or 312 pounds per square inch. 
When this value is compared with the pressure reading of 2,250 pounds 


158 


per square inch, it will be evident that the latter unit pressure was 
exerted over the equivalent of only a portion of the area in contact 
with the earth, in this case only 14 percent of that area or approxi¬ 
mately 19 square feet. In other words, while some upward pressure 
may be exerted over the entire area of the bottom slab in contact with 
the earth, the largest and most destructive pressures resulting from 
tamped explosions are exerted over a quite limited area of the floor 
slab nearest the explosion. While this may appear somewhat sur¬ 
prising, it is consistent with the evidently concentrated force exerted 
on the bottom of the slabs, as evidenced by the severe cracking in the 
centers of the slabs and the formation of definite craters in the concrete 
on the undersides of two of the floor slabs. If the cone of earth pres¬ 
sures from tamped explosions is as concentrated as this, the difficulty 
of attempting to predict total pressures or damage to underground 
structures, using earth pressures corresponding to given explosive 
charges and distances, will be evident. 

Scale Effect—Comparison of Models and Prototypes 

Instrumental results for the tamped explosion tests, particularly 
the shelter-displacement records, were not satisfactorily consistent. 
The over-all damage to the shelter floors and walls was, however, prac¬ 
tically identical in the two model scales, indicating that damage to 
the prototype structures under similar conditioning would be com¬ 
parable and predictable. In view of the scale reduction of accelera¬ 
tion values; i. e., prototype values, three-eighths those of the A shelters, 
the accuracy of the test results is considered sufficient to furnish the 
order of magnitude of the acceleration values with which it will be 
necessary to contend. In any case, both the acceleration values and 
the over-all damage from the explosion of 1,300 pounds of TNT only 
21 feet from floors of the prototype structures, are of such magnitude 
as to make it evident that bombs containing such amounts of explosive 
must be prevented from penetrating under and detonating so near to 
the shelter floors. 


PART V—CONCLUSIONS AND RECOMMENDATIONS 


IiMI»ACT rENETRATION RESULTS 
Impact Penetration Coeflieient 

The tests identify a reliable penetration coefficient in the formula 
S=kPV" of A:=0.0028 for a class E reinforced concrete having a 
nominal 28-day compressive strength of 3,000 and an actual of 4,500 
to 5,000 pounds per square inch. Required thicknesses of concrete 
of this strength to resist bomb or projectile penetration may be deter¬ 
mined, within satisfactory limits of accuracy, with the above formula 
and penetration coefficient. Penetration coefficients for concrete 
of other strengths and the effect generally of concrete strength upon 
penetration were not determined. Pending their determination, 
penetration may be assumed to vary in accordance with the observa¬ 
tions of Skramtajew, approximately as indicated in Table 24. 


Table 24 


Concrete strength, pounds 
per square inch_ 

2 , 000 

2,500 

3,000 

3 , 5001 

4,000 

; ! 1 

4 , 500 i 5,000 i 6,000 

7,000 

8.000 

9,000 

10,000 

Relative penetration_ 

1.00 

0 . 88 

0 . 775 

0 . 71 

0.66 

0.625 0.595 0.565 

0 . 545 

0 . 53 

0 . 51 

0.50 


Effect of Relative Slab Thickness 

With given projectiles and concrete, and assuming no deformation 
of projectiles at impact or during penetration, when the thickness 
of the reinforced concrete is at least 3.25 times the calculated pene¬ 
tration, the depth of penetration varies only with the sectional 
pressure, striking velocity and obliquity of impact; when the thick¬ 
ness of concrete lies between 3.25 and 2 times the above-stated pene¬ 
tration depth, the depth of penetration is also a function of the rela¬ 
tive concrete thickness; and when the concrete thickness is reduced 
to about twice the above-stated penetration depth, perforation may 
be expected to occur. The above-described phenomenon, which was 
discovered in the course of the tests, has an important bearing on the 
general theory of penetration in concrete and similar elastic friable 
materials having low tensile strengths compared with the compressive, 
because it reveals, irrespective of what particular penetration formula 
may be used, that the relative slab thickness as well as the other 
factors referred to, must be considered when attempting to calibrate 
penetration formulae or predict depths of penetration. 

( 159 ) 















160 


Reinforcing 

Conventional one-or two-way reinforcing, with usual bent stirrups 
and wired intersections, forms planes of cleavage in the concrete 
which promote scabbing, particularly in the direction of the trans¬ 
verse reinforcing if it is nearest the slab surface, and is generally 
unsatisfactory for resisting impact penetration and explosion effects. 
Special reinforcing as herein described, consisting of longitudinal steel 
welded to diagonal shear steel to form trusses, and transverse bars 
placed inside the longitudinal steel at each face, was found definitely 
superior to the conventional two-way reinforcing in limiting and 
reducing scabbing and holding the shattered concrete in place. 
There was also some evidence that this special reinforcing may have 
been effective in reducing the depth of penetration, but the tests were 
insufficient in number to warrant the general conclusion that, within 
usual percentages of steel used, the reinforcing can be so disposed as 
to reduce projectile penetration. 

Antiscabbing Plates 

Antiscabbing plates were found to be effective in increasing resist¬ 
ance to perforation, but since equivalent resistance to perforation can 
be provided at less cost by increasing the concrete thickness, and 
since the presence of plates underneath adds materially to the diffi¬ 
culty of inspecting and repairing the slabs when damaged, their use 
is not recommended, except for certain conditions for which roofs of 
single slab construction appear necessary or desirable. 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures 

Penetration results were satisfactorily consistent, reliable and 
definite. No significant scale effect in respect to penetration was 
found. When consideration is taken of the effect of slab thickness 
upon depth of penetration as described in Part IV, test results on the 
reduced scale models may be extended to the full-size prototype 
structures with the expectation that the results on the latter will 
correspond to those on the models in accordance with the scale ratio. 

EXPLOSION RESULTS 

Penetration Coeffieient and Limit Thicknesses 

Insufficient tests were made to calibrate the explosion penetration 
formula for limit thickness 

N (feet)=.^c'VC'(poiiM^ 

but the penetration coefficient determined from the one test which 
was suitable for this purpose, on slab C (c2), gave a value of c' = 0.23 



161 


for the 4,500 to 5,000-pound concrete used in the tests, which is in 
reasonably close agreement with the c' values given in ‘‘Civil Pro¬ 
tection,’’ of 0.20 for a 4,700 to 5,200-pound concrete, and 0.22 for a 
3,700 to 4,200-pound concrete. The c' value of 0.23 is considered 
satisfactory for design of structures in which class E concrete, with a 
nominal 28-day strength of 3,000 and an actual of 4,500 to 5,000 
pounds per square inch, is used. Additional tests are required to 
determine explosion penetration coefficients and limit thicknesses for 
concretes of other strengths. 

Earth Fill Between Slabs 

The tests show that an earth fill between slabs serves no useful 
purpose. It does not reduce penetration or scabbing of the upper 
slab, makes repairs difficult, adds useless weight on the slab and 
foundations, and increases the cost of the structure. If a bomb were 
to perforate the upper slab and detonate in the earth fill, a tamped 
explosion with aggravated damage results. If the bomb does not 
detonate, a hazardous problem is presented to locate and remove it 
from the earth fill. 

Reinforcing and Antiscabbing Plates 

Conventional two-way reinforcing was likewise unsatisfactory to 
resist explosion effects. The special reinforcing was definitely 
superior to it in limiting and reducing scabbing from explosions. 
Antiscabbing plates prevent rejection of the shattered concrete, but 
do not reduce the damage to the slab and have the disadvantages 
referred to under penetration results. 

Effect on Concrete Edges 

Because of the pronounced shattering effect on concrete edges by 
splinters and concrete fragments from near-by explosions, exterior 
corners and edges of bombproof structures should be chamfered or 
rounded. 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures 

Quantitative results from explosion tests were less definitive and 
uniform than those from impact penetration tests. There was some 
evidence of scale effect in the explosion tests on the (a), (b), and (c) 
shelters. However, since the tests showed that the earth fill type of 
double slab, in which the principal evidence of scale effect appeared, 
should in any case not be used, the evidence of scale effect in these 
cases, when considered in conjunction with the supplementary 
explosion tests on shelters and the explosion tests on the additional 


162 


slabs, does not alter the general conclusion that there is no indication 
of scale effect great enough to require any large factor of safety in 
extending the results of these tests to full scale design. 

INSTRUMENTAL RESULTS 
Single Slab Roofs 

Excessive displacement, initial velocity, acceleration and shock 
occur within the ceiling slab and at the surface of the ceiling when the 
explosive charge is detonated in contact with or within the ceiling itself 
as it may when the roof is composed of a single slab, or the upper slab 
of a double slab roof has been perforated. The upper slab should 
therefore be designed of such thiclaiess as to prevent impact perfora¬ 
tion. If the structural mtegrity of a single slab roof is impaired by 
bomb damage, it cannot readily or fully be restored to its original 
strength and condition. Single slab roofs are not considered suitable 
for use on personnel shelters, communication centers, power plants or 
other structures in which personnel or important and delicate instru¬ 
ments or equipment are to be housed, but may nevertheless be 
preferable for certain structures, such as imderground hangars or for 
pump wells and machinery chambers in which head room is re¬ 
stricted. Where conditions require the use of single slab roofs, instru¬ 
ments and equipment attached to them should be secured with shock- 
proof mountings. 

Increased Air Pressure Inside Shelters 

The tests do not indicate any cause for concern that concussion waves 
in the ceiling slab of a shelter from bomb impact and explosion will 
result in an increase in air pressure inside the shelter which will be 
harmful to personnel. Because of the increased am pressm-e which 
may be transmitted through the doors to the inside of the shelter from 
bomb explosions outside, shelter doors should be kept closed during 
bombing raids. 

Instruments Recommended for Future Ex|>losion Tests ^ 

The blast gages within the chamber, particularly the quartz piezo 
gage and the Williams gage, are recommended for tests of air blast 
condition within the chamber. Ceiling deflection gages should be 
used wherever there is interest in magnitude of disturbance short of 
that for complete failure of the structure. Gages will be damaged, of 
course, when the disturbance is great enough to produce rupture of 
the slab. These gages can be set up to give displacement time curves, 
and reliable estimates can then be made of the orders of velocity and 
acceleration. Strain gages of the manganin wire type employing the 
spool mounting, are apparently satisfactory for work in concrete. 


1 From Naval Proving Ground report. 



163 


Time records can be obtained by oscillograph, and the sign of the strain 
is evident at any phase. The results of the present tests are not con¬ 
clusive as to the reliability of these gages, since no reliable check was 
possible, but there appears to be no reason for questioning their 
significance. Further tests of the gages in concrete should be made 
imder conditions where strain can be checked by alternative methods. 
The Hopkinson bar does not give enough additional information to 
justify its use for measuring shock intensity where the medium is 
subject to appreciable displacement as in the ceiling slab under the 
detonation. In the absence of body motion, the bar can probably be 
used to measure the compression from shock even in the heavy medium, 
as it is used in air. But in a slab receiving appreciable initial velocity, 
the bar gages measure shock intensity as such only indirectly with the 
aid of bt values, calculated or measured, as defined by mean accelera¬ 
tion. A special bar having a time piece of extremely small mass 
might distinguish the shock pressures, since the velocity of the time 
piece imparted by shock, increases with decreasing mass, provided 
the time for the wave to travel across the time piece is not decreased 
too much as the mass is decreased, while the velocity from initial dis¬ 
placement of the slab remains unchanged. It would be better to 
observe the motion of the time piece without introducing springs. 
Altogether, the instrument is not well adapted for the purposes of 
the subject tests. 

OVEK-ALL RESULTS 

The results of impact penetration and explosion tests on shelters 
and additional slabs are considered satisfactorily definite with respect 
to the merits of the alternative types of roof construction tested and 
the degree of protection required. 

Double Slab Roofs 

A full-scale structure with a double-slab roof, having a vented air 
space between the slabs, offers the best protection against high 
explosive bombs, provided the upper slab is of sufficient thickness to 
prevent impact perforation prior to detonation. A 6-foot thickness 
of upper slab, constructed of class E concrete having a nominal com¬ 
pressive strength of 3,000 and an actual of 4,500 to 5,000 pounds 
per square inch, will prevent perforation by a 2,000-pound AP bomb 
of a 22 percent charge-weight ratio, striking at 1,000 feet per second 
with 20° obliquity. A 4-foot lower slab, specially reinforced, is 
required to carry the design live and dead loads, the shoring and 
other loads during construction, to resist the blast and fragmentation 
effects from explosions which take place within and near the bottom 
of the upper roof slab, and, finally, to resist, to a reasonable degree, 
the explosion effects of any bombs which, because of larger size or 


164 


hiorlier striking velocities, perforate the upper slab or in other manner 
are able to reach the lower slab before detonation. 

Reinforcing 

Special reinforcing should be used in place of the conventional 
two-way reinforcing with bent stirrups and wired intersections. It 
should consist of longitudinal bars welded to diagonal shear bars 
to form trusses, and transverse bars placed inside the longitudinal 
bars at each face, or any other arrangements found equally effective. 

Protection Against Demolition Bombs 

A single or double-slab roof designed to resist the impact penetra¬ 
tion and explosion effects of a 2,000-pound AP bomb will afford an 
ample margin of protection against a demolition bomb of the same 
weight, notwithstanding the much greater explosive content of the 
latter; or, stated in another way, will afford equal protection against 
a much larger bomb of the demolition type. This is true because of the 
smaller sectional pressure and penetration of the demolition bomb as 
compared with the armor-piercing type of bomb. 

Protection Against Larger Bombs or Higher Striking Velocities 
Than the Basic 

If desired, the upper roof slab can be made secure against larger 
bombs than the basic, 2,000-pound AP bombs, or higher striking 
velocities than 1,000 feet per second, by appropriately increasing 
the thickness of the upper slab in accordance with the penetration 
formula and constant herein described. For a reasonable increase 
in bomb size, the 4-foot thickness of lower roof slab will not be affected. 

Antiscabbing Plates 

Because of the increased cost as compared with additional thick¬ 
ness of concrete and increased difficulty of inspecting and making 
repairs to damaged slabs, the use of antiscabbing plates is not recom¬ 
mended, except with single slab roofs where the protected area is 
relatively small, the head room is restricted, or it is important that 
shattered or pulverized concrete be prevented from falling into the 
protected space. 

Shelter Entrances 

Indirect entrances should be provided into bombproof shelters so 
that entrance doors will not be directly exposed to blast and splinter 
effects from bombs which strike in the vicinity. Doors should be 
strong enough to withstand blast pressures. 


165 


TAMTEl) EXPLOSION RESULTS 
Over-all Damage 

Shock and over-all damage to the shelters and instruments from the 
tamped explosion tests, were found to be severe, and would be equally 
damaging to prototype structures and personnel and equipment 
within, if explosive charges corresponding to light-case 2,000-pound 
bombs containing as much as 65 percent (1,300 pounds) of TNT 
were permitted to detonate appreciably nearer to the floors than the 
camouflet radius distance for the particular explosive charge, in this 
case 32 feet. Test explosions corresponding to a prototype distance 
of 21 feet under the shelter floors were very destructive. The neces¬ 
sity of protecting the shelter floor slabs from at least the heavier 
tamped explosions is indicated. The damage to floor slabs is some¬ 
what distributed, but comparable to the results of a heavy con¬ 
centrated impact loading. Judging from the accentuated cracking 
in the floor slabs, and the formation of craters in the concrete on the 
underside of two of the slabs, the cone of earth pressures from tamped 
explosions is evidently concentrated over a relatively small area of 
the slab nearest the explosion. Because of the large negative mo¬ 
ments induced in the corners between the side walls and floor by the 
explosion underneath, which resulted in cracking and breaking of the 
concrete, heavy fillets in the concrete and continuous reinforcing at 
these corners are required. Cold joints during construction should 
be avoided in these locations. 

Instrumental Results 

Earth pressures from the tamped explosions were of high order 
but of very short duration, characteristic of explosions. Both slab 
and shelter accelerations were likewise of a high order and, while not 
entirely consistent, gave an idea of the values with which it is necessary 
to contend. The violence of the shock and resulting high accelera¬ 
tions demonstrate the necessity of preventing tamped explosions of as 
large as 2,000-pound light case bombs of a 65 percent charge-weight 
ratio, under and nearer shelter floors, than the corresponding camouflet 
radius; namely, 32 feet. 

Scale Effect—Application of Model Tests to Full-scale Prototype 

Structures 

Over-all damage to shelters of both scales, as well as the formation 
of craters and camouflets, were uniformly consistent with the amounts 
and calculated positions of explosive, with no evidence of scale effect, 
thereby confirming the validity of the test conditions selected for 
similitude and, that test results might with confidence be extended 

420.-i04° 41——12 


166 


to the full size prototype structure, in accordance with the scale 
ratio. Earth pressure readings were, likewise, reasonably consistent 
between themselves and in expected magnitude, based on the explo¬ 
sive charges used. Slab accelerations and shelter displacements were 
not consistent. The latter in particular were widely at variance 
with expected results. Both acceleration and displacement readings 
were, however, of value in indicating the order of magnitude to be 
expected. For explosions of 1,300 pounds of TNT 32 feet under the 
prototype structure, a slab acceleration of perhaps 15g may be 
expected and an acceleration of 2g for the structure as a whole. 
For similar explosions at a distance of 21 feet, the corresponding 
accelerations might be 34g and 4.5g, respectively. Vertical dis¬ 
placements of the prototype structure might under these conditions 
be about 1.3 inches and 3 inches. 

Protective Apron 

A convenient and perhaps obvious means of preventing penetration 
and detonation of bombs alongside and under bombproof structures 
is a protective apron of reinforced concrete flush with the ground 
surface around the structure. Such an apron should be of sufficient 
thickness to prevent perforation by bombs up to 2,000 pounds of 
the light case type containing as much as 65 percent (1,300 pounds) 
of explosive filler. It should also be of sufficient width to prevent 
penetration from any probable angle of approach, under the apron 
and shelter, nearer to the floor slab than the radius of camouflet for 
the given explosive charge, in this case 32 feet. Provision of suffi¬ 
cient slab thickness to prevent perforation by bombs of the heavy 
case type in the larger sizes will probably not be warranted. This 
type of bomb contains only one-fourth to one-third as much explo¬ 
sive as the light case bombs, with correspondingly reduced destruc¬ 
tive effect. Furthermore, the floor slab must be of substantial thick¬ 
ness to support the static loads, and can, at little additional cost, be 
designed to withstand tamped explosions from this type of bomb. 
Medium and light case bombs would detonate upon striking the apron, 
if equipped with instantaneous fuses, otherwise would deform or 
break up with low order detonation. Heavy case bombs of the larger 
sizes, if fitted with delayed action fuses, would perforate the apron, 
but with reduced residual velocity and penetration of the earth below, 
and, because of the greatly reduced explosive content, would result in 
much less destructive effect on the shelters. 

Protection of Equipment Against Tamped Explosion Effects 

Since bombproof structures may be subjected to the shock from at 
least the less severe tamped explosions of bombs of the smaller sizes 
or explosive content, shockproof mountings should be provided for 


167 


equipment and piping housed in the structures. It is expected that 
personnel would be able to survive these less severe explosions, with¬ 
out harmful physiological or psychological effects. 

RECOMIVIENDED ADDITIONAL TESTS 

Additional tests are required to determine penetration coefficieuts 
for concretes of various strengths and densities, at projectile strikiug 
velocities up to 1,300 feet per second; to determine proof thicknesses 
of concrete of various strengths against explosions; to obtain further 
data on the effect of various arrangements and percentages of rein¬ 
forcing steel on scabbing and penetration; also to determine if prac¬ 
ticable the optimum arrangement and minimum amount of reinforcing 
needed; to determine the effects of blast pressures on a number of 
types of shelter doors; and to obtain additional data on earth pres¬ 
sures and thicknesses of concrete required to resist the effects of 
tamped explosions against floors and underground walls of shelters. 
Because of uncertainties inherent in small caliber tests, it is considered 
that penetration tests should be made with not smaller than 3-inch 
projectiles. 

RECOIVIIMENDED SIZE OF SLABS FOR FUTURE PENETRATION AND 
EXPLOSION TESTS 

In future penetration tests, it is likely that with slabs of reinforced 
concrete, mounted as armor plates are mounted for test, the impacts 
can be placed within 15 calibers of the edges of the slab without 
producing abnormal cracks or fracture. For detonation tests, the 
flat dimensions of the slab should be at least 35 calibers. In other 
words, slabs for penetration tests should be not less than 4 feet 
square if 3-inch projectiles are used, and 8 feet square for 6-inch 
projectiles; and 9 feet and 18 feet square, respectively, for the corre¬ 
sponding explosion tests, although for the latter, and for tests in general 
to determine resistance to penetration and explosion effects of high 
explosive bombs, test structures simulating actual bombproof struc¬ 
tures are considered superior to flat slabs. If the penetration tests are 
for the purpose of determining penetration constants and calibrating a 
penetration formula, the slabs should have a thickness 3.25 or prefer¬ 
ably 3.50 times the calculated penetration depth in a slab of great 
thickness. For a concrete such as was used in the shelters and slabs, 
a 3-inch 13-pound projectile, a striking velocity of 1,000 feet per 
second and 20° obliquity, the required test slab thickness would be 
20 inches or preferably 21 inches; for a striking velocity of 1,300 feet 
per second, the preferred thickness would be 26 inches. For concretes 
of other strengths, the thicknesses would vary in a ratio not yet 
determined but thought to be approximately as indicated in Table 24. 


PART VI—DESIGN OF A BOMBPROOF 
STRUCTURE 


The application of the test results and conclusions, to the design of 
a typical bombproof structure is briefly described hereinafter. Tlu' 
structural design of bombproof structures is sometimes thought to 
consist principally of the design of the roof. While the roof doubtless 
receives most of the bomb hits and constitutes, therefore, an important 
part of the structure, the structure is also subjected to and must, 
therefore, be designed for other conditions and forces than the pene¬ 
tration and explosion effects on the roof. Side walls may be subjected 
to oblique impact of bombs or shell fire, and the floors and side walls 
which are below ground, to heavy earth pressures from tamped 
explosions of bombs which penetrate near or under the structure. 
Entrance doors must withstand blast and splinter effects of bombs 
which detonate nearby. For convenience of discussion, the designs 
of the component parts of the structure for these various conditions 
are considered separately. 

UPPER ROOF SLABS 

The upper slab of a double slab roof must be designed to support 
its own weight and any static live loads which may be imposed upon 
it. Its most important function, however, for which it is primarily 
designed, is to resist perforation by the largest bomb which it is 
assumed may strike it, in order that subsequent detonation will not 
occur in contact with the lower or ceiling slab. It is expected that 
detonation will occur within and near the bottom of the upper slab 
and that it will accordingly sustain heavy damage and require exten¬ 
sive repairs, but the shock and concussion waves will in large part 
not be transmitted through the lower slab into the shelter, and the 
lower slab will remain intact and structurally sound. 

Required Thiekness of Upper Roof Slabs 

The proof thickness required to resist perforation by a given pro¬ 
jectile and striking velocity may be determined from the impact 
penetration formula S {ieet)=kPV", and the factors appropriate to 
the assumed conditions. For example, the proof thickness required 

(168) 


1G9 


just to ])r('vent perforation of a class concrete having a nominal 
28-(lay strength of 3,000 and an actual of 4,500 to 5,000 pounds per 
square inch, by the basic 2,000-pound AP l)oml) striking at 1,000 feet 
per second with an oblicpiity of 20°, would be twice ^ the depth of 
peiu'tration given by the above formula, adjusted for the obliquity 
of impact. 

Proof thickness=2 kPV" cos 6, if 6 is the angle of obliquity. (See 
Table 25 for cosines.) 

= 2X.0028X1432X.752X.94 
= 5.67 feet or 68 inches. 

A six foot thickness should be used. 


Table 25.— Cosines of angles 


Degrees 

Cosine 

Degrees 

Cosine 

f Degrees 

Cosine 

1 

1.000 

11 

0. 982 

21 

0. 934 

2 

.999 

12 

.978 

22 

.927 

3 

. 999 

13 

.974 

23 

.920 

4 

.998 

14 

.970 

24 

.914 

5 

. 996 

15 

.965 

25 

.906 

6 

.995 

16 

.961 

26 

.899 

7 

.993 

17 

.956 

27 

.891 

8 

.990 

18 

.951 

28 

.883 

9 

.988 

19 

.946 

29 

.875 

10 

.985 

20 

.940 

30 

1 

.866 


Degrees 

Cosine 

1 

I Degrees 

Cosine i 

i Degrees 

Cosine 

31 

0.857 

41 

0. 755 

51 

0. 629 

32 

.848 

42 

. 743 

52 

.616 

33 

.839 

43 

.731 

53 

. 602 

34 

.829 

44 

. 719 

54 

. 588 

35 

.819 

45 

.707 

55 

.574 

36 

.809 

46 

.695 

56 

.559 

37 

. 799 

47 

.682 

57 

.545 

38 

.788 

48 

.669 

58 

.530 

39 

. 777 

49 

.656 

59 

.515 

40 

.766 

50 

.643 

60 

.500 


Increased Protection 

If the striking velocity were increased to 1,300 feet per second, 
which is near the maximum or tei-minal v(4ocity requiring an altitude 
at release of about 30,000 feet, a proof thickness of 85.5 inches would 
be required, and if at the same time the bomb were increased from 
2,000 to 3,000 pounds, a total proof thickness of upper roof slab of 
of 98 inches would be required to prevent perforation. Since the 
difficulties of handling are increased, and the effective destruction 
per unit weight of explosive is decreased, as the size of the bomb is 
increased, it is probable that for some time in the future, 2,000 pounds 
will be the largest AP bomb against which protection will need to 
be provided, and, therefore, except for special conditions, that a 
6-foot thickness of upper roof slab will provide adequate protection. 

1 This is the class of concrete used in the test shelters and recommended for use in bombproof structures. 
It consists approximately of the following quantities per cubic yard, for 1 inch maximum size of coarse 
aggregates: 

6.3 sacks of cement, 

6.2.5 gallons of water per sack of cement including the free water in th(> aggregates, 

186 pounds of fine aggregates and 
304 pounds of coarse aggregates. 

The fine aggregates should comprise 34 to 42 perccmt of the total aggregates by weight . 

2 In accordance with the findings given in the discussion of the effect of relative slab thickness upon 
penetration. 






























170 


Vent Oj>eniiigs Between Roof Slabs 

Vent openings not less than 5 feet wide should he provided in the 
side walls between the roof slabs to permit the escape of the gases of 
explosion, should perforation of the upper slab occur, and to permit 
inspection and repairs of the underside of the upper slab when 
damaged. There is some question whether vent openings are fully 
effective in reducing the air pressure because of the extremely high 
speed of explosions, but the openings are required in any case for 
the other reasons given. They obviously make the space between 
the slabs more useable for peace time purposes. 

Overhang of Upper Roof Slabs 

Since the primary function of the upper roof slab is to prevent 
bombs from reaching and detonating against the lower ceiling slab, 
this slab must not only be of sufficient thickness to prevent perforation, 
but also have sufficient overhang to make unlikely the entrance of an 
obliquely descending bomb through one of the vent openings between 
the slabs. The amount of overhang is a matter of judgment based 
on usual angles of descent of bombs from various altitudes and 
horizontal speeds. Not less than a 5-foot overhang is recommended. 
With a vertical clearance between slabs of 8 feet, such overhang would 
prevent any bomb descending at a greater angle to the horizontal 
than 58° from entering through one of the openings. As projectiles 
from shell fire at close range would have flat trajectories, it would be 
advisable to sandbag all openings toward the open sea; and it might 
be prudent, if air attack is expected, to sandbag all vent openings 
between slabs to prevent the chance entry of a bomb from a low 
flying plane. 


Reinforcing 

The reinforcing for the roof slabs, side walls, floors and apron, 
is of the special type used in the additional test slabs, consisting of 
longitudinal steel in each face welded to continuous diagonal shear 
bars to form trusses, and of straight transverse bars placed inside the 
longitudinal steel in each face, to resist the tendency to bend outward 
and strip off the concrete covering. The details are shown in Fig. 
153. The trusses would be reversed in the floor slabs, with the 
heavier bars at the top. This type of reinforcing is easily fabricated 
and erected. The trusses are manufactured by bending the diagonal 
shear steel over a jig, spot welding the longitudinal bars to same at 
top and bottom, and completing the welding after removal from the 
jig. The points of bending of the diagonal bars serve as convenient 
guides for attachment of the straight transverse bars thus eliminating 
the need for additional chairs or spacers. Reinforcing can best be 


171 



TYPICAL LONGITUDINAL TRUSS 


EXTERIOR FACE OF WALLS OR TOP FACE 
OF ROOF AND CEILING SLABS—\ 



transverse rods 

‘ ABT. 6"C.C. 
ADDITIONAL SHORT RODS <g) 

e-c.c. 


-SIZE OF THESE TRANSVERSE RODS VARIES IN ACCORDANCE 
WITH DEAD AND LIVE LOAD DESIGN STRESSES. SPACING ABT. 6'' 



SPACER BARS <§) I’-S’- 


ARRANGEMENT OF TYPICAL 
REINFORCING 


ARRANGEMENT OF ADDITIONAL 
TRANSVERSE REINF. 


Fig. 153.—Reinforcing for bombproof structures. 


assembled in sections and lifted into place as a unit. To facilitate 
assembly of the reinforcing steel and inspection of the placement of 
the concrete in the deep slabs, to assure free flow of the concrete and 
thorough embedment of the reinforcing, and to prevent arching of 
the aggregates and formation of honeycomb, a minimum spacing of 
the reinforcing steel in each direction, of 6 inches was adopted. 
Where this spacing results in bars larger than 1 inch square, they are 
disposed in two layers. This is likewise the maximum spacing, as 
close spacing and smaller bars are desirable in order to minimize 
scabbing. Any closer spacing than 6 inches, however, would not only 
have the practical disadvantages enumerated above but would in 
effect form a mesh which would create a plane of cleavage in the 
concrete and promote scabbing. 

The amount of reinforcing used was determined in part by the 
usual structural requirements to carry the static live and dead loads 
imposed upon the structure. The remainder was fixed arbitrarily 
from the test results influenced by practical and economic considera¬ 
tions. The amount of transverse steel in the bottom of the roof and 
ceiling slabs and in the top near the supports, was determined by 
analysis of the section as a rigid frame bent. An arbitrary size of 
%-inch round bars was adopted for the transverse bars in the top of 
the slab and for the longitudinal truss bars top and bottom, as being 
the smallest size which could readily be handled in long lengths 
without deforming. The diagonal shear bars in the longitudinal 


































172 


trusses were arbitrarily made K-iiich <f). The amount of transverse 
steel in the bottom of the slab, and also the total percentage by 
volume of steel used, are dependent upon the slab span. The amount 
and percentages required for both 6-foot and 4-foot thick slabs, for 
various roof spans are given in Table 26. It will be noted, for a 
6-foot slab that the amount varies from 74 to 125 pounds per cubic 
yard of concrete and for a 4-foot slab from 92 to 170 pounds. These 
minimum recommended amounts may be compared with 54 pounds 
of steel per cubic yard of concrete which is given in one British publica¬ 
tion as a satisfactory amount. The percentages by volume of 0.55 
to 0.85 percent for the 6-foot slab and 0.70 to 1.13 percent for the 
4-foot slab may also be compared with usual percentages of slab 
reinforcing of 1.0 to 1.5 percent. 


Table 26.' —Reinforcing steel for bombproof structures 


Clear span 2 
slab 

1 

Reinforcing in bot¬ 
tom of slab per 

Reinforcing in bottom face 

Total reinforcing 


Thickness 

linear foot of slab 

Percent by 

1 volume 

1 

Weight per 
cubic foot of 

Percent by 
volume 

Weight i^er 
cubic foot of 



Trans¬ 

verse 

Longitu¬ 

dinal 

concrete 

concrete 

6 feet 

4 feet 

6 feet 

4 feet 

1 0 feet 

4 feet 

6 feet 

4 feet 

6 feet 

1 4 feet 

Feet 

80 

Feet 

1 14 


i 

0.142 

0.212 ! 

0.70 

1.04 ' 

0. 556 

0.696 

2.74 

3.42 

39 

19 


2-5^0 

. 173 

.259 

.86 

1.28 

.585 

.743 

2.90 

3. 65 

47 

24 

2-J4<t> 

2-H<t> 

.210 

.314 

1.03 

1.52 

.622 

.798 

3.07 

3.92 

54 

29 

2-l<t> 


.254 

.380 

1.24 

1.85 

.666 

.864 

3.28 

4. 22 


34 

2-1 □ 


.302 

.453 

1.48 

2.22 

.714 

.937 

3.52 

4.60 


37 

i-Vs<t> 


.349 

.522 

1.71 

2.56 

.763 

1.006 

3.75 

4. 94 


44 

4-1 </) 

2-y<f> 

.437 

. 054 

2.13 

3. 18 

.851 

1. 138 

4.17 

5. 56 


51 

4-1 □ 


.533 

.800 

2.61 

3.92 

.947 

1.284 

4.65 

6. 30 


• Includes \^<i> shear reinforcing (=l.:i4 pounds i)er cutiic foot, or 0.272 percent). 
2 Single-bay two-story bent. 


LOWKK l{()OF SLABS 

The lower roof slab must be designed to withstand the fragmenta¬ 
tion and blast effects of explosions occurring in the slab above, and to 
support the resultant debris from the same source. The section is 
first analyzed as a completed bent for the dead load of the slabs, a 
roof load appropriate to the climate and a lOO-pound storage live 
load on the ceiling; then as a partial bent for stresses in the ceiling slab 
and side walls assuming the ceiling to support the shoring and weight of 
freshly poured concrete used in making repairs to the upper slab 
when damaged. If the resultant steel stresses are within the yield 
range, no additional steel is provided. And, finally, the slab must 
be designed—and this may be the controlling factor—to withstand 
the explosion of bombs which because of higher sectional pressures 
or striking velocities than the upper slab has been designed to with¬ 
stand, or because of two or more hits in the same spot, are able to 
perforate the upper slab and detonate against the lower or ceiling 
































173 


slab. It will be evident that the lower slab is more than a fragment 
ca teller. 

Ketjuired Thickness of Lower Koof Slabs 

The proof thickness of reinforced concrete required just to prevent 
perforation by the amount of explosive filler in the basic 2,000-pound 
AP bomb of a 22 percent charge-weight ratio; namely, 440 pounds of 
TNT, is given by the formula 

^^(feet) =2c' VC' (pounds) 

= 2X.23y^ 

= 3.5 feet. 

It will be evident that the adopted 4-foot thickness of lower slab 
provides a satisfactory margin of protection against the explosion of 
the basic bomb in contact with the slab. 

SINGLE-SLAB KOOFS 

A single-slab roof where used must be desgned to support its own 
weight and the live loads referred to, and to withstand the impact 
penetration and explosion effects of the largest assumed size of bomb, 
with reasonable assurance that perforation or scabbing will not occur. 
Aside from the difference in transmission of shock and concussion, the 
essential difference between double and single-slab roofs is that in the 
former the upper slab may be repeatedly damaged and repaired 
without impairment of the lower structural slab or reduction in the 
degree of protection afforded, whereas when a single-slab roof, which 
has the dual role of protective medium and structural support, is hit 
and damaged, the structural integrity is impaired, and perhaps can¬ 
not readily or completely be restored. With double slabs, repairs may 
be made to the upper slab when damaged without interference with 
activities in the protected space below the lower slab. From a con¬ 
struction standpoint, the double slab is also preferable as two slabs of 
moderate thickness can be more easily and better built than one slab 
of their combined thickness. Nevertheless, as previously mentioned, 
there may be conditions for which a single slab will be preferable to 
the double slab. 


Required Thickness of Single Slabs 

The required thickness of single slab may be determined by adding 
the proof thickness required to resist impact perforation to the proof 
thickness required to resist explosion perforation. For the basic 
2,000-pound AP bomb, as determined above, the thickness would be 
5.67 feet plus 3.5 feet or a total of 9.17 feet. However, it will be 
recalled from the discussion of the effect of relative slab thickness 
upon the depth of penetration, that when the ratio of slab thickness 
to depth of penetration falls below 3.25, the phenomenon of increased 



174 


penetration comes into play. Hence the ratio must be determined. 
In this case the ratio is found to be 9.17/2.835 or 3.23, so that no 
increase in penetration would be expected. It would, nevertheless, 
be prudent to increase the slab thickness to 10 feet, not only to bring- 
the ratio above 3.25 but also to provide a measure of reserve protection 
against perforation or scabbing. This thickness of 10 feet, it will be 
noted, is the same as the combined thickness of the double slabs. The 
latter, however, provide a considerably greater margin of safety, in 
addition to superior protection against shock and concussion. If a 
single-slab roof is used, any delicate instruments, equipment and 
piping which it is necessary to attach to same should be secured with 
shockproof mountings. This same precaution applies to side walls. 

sidp: walls 

The side walls of bombproof structures must be designed to support 
the roof structure and the loads imposed upon it, and to resist oblique 
impact of bombs, shell fire, and blast, splinter and fragmentation 
effects of explosions nearby. If below ground, they must be designed 
to withstand the heavy earth pressures resulting from tamped explo¬ 
sions of bombs which penetrate the ground adjacent to the sidewalls 
of the structures. 

Required Thickness of Side alls Above Ground 

The 4-foot thickness chosen for side walls will prevent impact per¬ 
foration of the basic 2,000-pound AP bomb arriving at any probable 
combination of obliquity and striking velocity. It will just prevent 
perforation, if this basic bomb is released at any altitude by a plane 
flying horizontally at 450 miles per hour. 

The 4-foot side walls and roofs have also been investigated with 
reference to protection against 6-inch and 8-inch shell fire from ships. 
The calculated penetration values are tabulated in Table 27. 

Since proof thicknesses are just twice the calculated penetration 
values given in the last two columns, it will be noted that the 4-foot 
side walls will afford protection against individual hits of 6-inch pro¬ 
jectiles even at point blank range and also against 8-inch projectiles 
at ranges over 12,000 yards. It would be advisable to increase the 
thickness of side walls exposed to shell fire from the sea to 6 feet. The 
roofs have an ample margin of protection against these projectiles at 
any range. 


Table 27 .—Effect of shellfire on bombproof structures 

PROJECTILE 6-INCH, 47 CAL., 105 POUNDS. SECTIONAL PRESSURE 534.7 POUNDS PER 

SQUARE FOOT 


Rangrc (yards) 

Striking 

velocity 

Angle of 
impact 
to hor. 

Obliquity of impact 

Penetration in 
inches 

Roof 

Cos d 

Side wall 

Cos B 

Roof 

Sidewall 

24,0(K) 

F.s. 

1,107 
1,078 
1,285 

1, 882 

o 

48.8 
30.0 

12.8 
3.6 

o 

41.2 

60.0 

0. 7524 
.5000 

0 

48.8 
30.0 

12.8 
3.6 

0.6587 

.8660 

.9751 

.9980 

11.25 
6.67 

9. 71 
12.64 
16. 51 
22.29 

18,()()() .. 

12,000 _ 

6,000 _ 










PROJECTILE 8-INCH, 55 CAL., 260 POUNDS. SECTIONAL PRESSURE 745 POUNDS PER 

SQUARE FOOT 


Rang*' (yards) 

Striking 

velocity 

Angle of 
impact 


Obliquity of impact 


Penetration in 
inches 

to hor. 

Roof 

Cos 6 1 

[ 

Side wall 

Cos B 

Roof 

Side wall 

2S,(XX) .. 

F.s. 

1,203 

48.7 

o 

41.3 

0. 7513 

48.7 

0. 6600 

17.05 

14.88 

24,000 _ 

1,128 

1,105 
1,362 
1,928 

46.6 

43.4 

.7266 

46.6 

.6871 

15.45 

14. 49 

18,0(X) 

28.2 

61.8 

.4726 

28.2 

.8813 

8.90 

18. 35 

12 000 

12.0 


12.0 

.9781 


24.18 

6,000 

3.6 



3.6 

.9980 


31.54 








Required Thickness of Side W alls Below Ground 

While tlie majority of bombproof structures at Naval Stations will, 
by reason of the ground water elevation and also by choice, be built 
above ground, some few will be built against hillsides with one or 
more side walls below ground and must be designed to resist tamped 
e.xplosions. A protective apron should be provided to prevent large 
size light case bombs with a high percentage of explosive from pene¬ 
trating and detonating along side walls below ground. To provide 
increased shock resistance against the smaller explosive charges of the 
heavy case bombs which are able to penetrate the protective apron, 
side walls below ground should be increased 50 percent or to 6 feet 
in thickness,‘and, if practicable, no piping or equipment should be 
attached to same. 

IMtOTKCTIVE APRONS 

The heavy earth pressures, shock and acceleration transmitted and 
the severe damage caused to structures by tamped explosions of bombs 
whicli penetrate and detonate under the floors of the structures, indi¬ 
cate the necessity of preventing such penetration at least by the larger 
size light case bombs which contain a high percentage of explosive 
filler. This can most effectively be done by heavy protective aprons 
around the structure, constructed of reinforced concrete. 

Required Thickness of Protective Aprons 

The proof thickness of protective aprons to prevent penetration by 
a 2,000-pound light case bomb of 65 percent charge-weight ratio, a 

















































176 


diameter of 24 inelies, and a sectional pressure of 630 pounds per 
square foot, may be determined from tb(' penetration formula. 

S=2 kPV" or 

S=2 kPV" cos^ for oblique impact. 

Using the values 0.0028 for the penetration constant ‘^k”; 630 for the 
sectional pressure ‘‘ P”; 0.752 for V" corresponding to a striking velocity 
of 1,000 feet per second; and an obliquity of impact of 35°, we find 
the required proof thickness to be 2.17 feet. For a 2,000-pound gen¬ 
eral purpose bomb of a 47.6 joercent charge-weight ratio, a diameter of 
19 inches and a sectional pressure of 1,016 pounds per square foot, at 
the same obliquity of impact; nanudy, 35°, the required proof thick¬ 
ness would be 3.5 feet. A thickness of protective apron was adopted 
varying from 3 feet 6 inches next to the shelter, to 3 feet at 
the outer edge. With a 6-inch step down from the floor of the shelter 
nnd a 6-inch drainage slope away from the shelter, the bottom of the 
protective apron comes level with the bottom of the 4-foot floor slab. 

Required Width of Protective Aprons 

The calculated penetration of the 2,000-pound light case and general 
purpose bombs, referred to above, into sandy soil, with an assumed 
penetration constant of 0.0367 would be 17.4 and 28.0 feet, respectively. 
Since these are distances along an oblique path, the conclusion was 
reached that an apron width of 20 feet would afford adquate 
protection. 

Residual Velocity 

Since provision of sufficient thickness of protective apron to prevent 
perforation by AP and SAP bombs was not considered warranted, the 
effect of such bombs which would penetrate the slab should be deter¬ 
mined. A delayed-action bomb would penetrate the earth for a 
distance dependent upon the velocity remaining after perforation of 
the protective apron. This residual velocity may be determined from 
tlie expression 

TV=TV'(l-|), or 

for oblique impact, in which 

1 ^ 2 ''=-. velocity factor corresponding to the residual velocity in 
feet per second, 

= velocity factor corresponding to the original striking 
velocity in feet per second, 
thickness of the concrete slab, 
s = depth of penetration in a slab of great thickness. 



A 2,000-poiind AP bomb similar to the prototype projectile, containing’ 
440 pounds of explosive and having a sectional pressure of 1,432 pounds 
per square foot, would penetrate 3.02 feet into concrete of unlimited 
thickness having a penetration constant of 0.0028. Assuming an 
obliquity of impact of 35°, by substitution in the above formula, we 
obtain 

K'- 2 X3.olx(K81i. ) 

= 0.219, 

corresponding to a residual velocity of 370 feet per second. See Fig. 
2. At this velocity, penetration of the bomb in the earth below the 
slab may be obtained from the penetration formula 

,S=0.0367X1432X0.219 
= 11.5 feet on the diagonal path. 

The detonation might, accordingly, occur under the shelter in a vertical 
line just inside the side wall where, however, the concussion would 
cause a minimum of damage to the floor slab. 

FLOOR SLABS 

Floor slabs must be designed to support the dead and live loads 
imposed upon them and to resist the heavy earth pressures and con¬ 
cussion from tamp(‘d explosions of obliquely falling bombs which are 
able to perforate the })rotective apron and detonate under the shelter 
floor. 

Required Thiekness of Floor Slabs 

It is believed that the 4-foot thickness of floor slab adopted will 
satisfactorily resist these forces. The magnitude of the forces and 
damage from tamped explosions is a function of the amount of explo¬ 
sive and distance from the center of explosion. From the test results, 
it is believed that earth pressures and concussion at a distance from th(‘ 
center of explosion at least equal to the caniouflet radius for the particu¬ 
lar explosive charge can be resisted without serious damage. Camou- 
flet radii may be calculated from the following approximate formula 

in which 

rc = radius of camouflet in feet 

(7= weight of explosive charge in pounds. 

For 2,000-pound light case, general purpose, and heavy case bombs 
containing respectively 1,300, 952, and 440 pounds of high explosive, 
the corresponding camouflet radii would be 32, 29, and 22 feet. 

Damage to shelter floors may be severe from tamped explosions, 
but if protective aprons are used, the heavier tamped explosions are 
unlikely to occur umhu- the shelters, because the lighter case bombs 
will be stopped b}- the aprons, and necessary perforation velocities for 



178 


the heavier case bombs are only a ttained by bomb release at the higher 
altitudes from which the angle of impact would too nearly approach 
the vertical to be able to penetrate under the structure. 

Supports and Mountings for Machinery and Equipment 

In addition to the shockproof mountings recommended for instru¬ 
ments, equipment, and piping attached to single roof slabs and side 
walls, supports for machinery and equipment mounted on the floor 
slab should also be constructed of elastic materials and otherwise 
cushioned against severe shock from below. Welded steel rather than 
concrete supports should be provided for turbo generators, compres¬ 
sors, condensers, and boilers, and welded steel machinery bases fa¬ 
vored over cast iron. One type of small cushioned steel machinery 
base and a detail of the rubber cushion support are shown in Fig. 
154. Switchboards, radio and other delicate equipment should be 
mounted on rubber or steel springs. Adequate foundation bolts 
should be provided. Piping should be steel rather than cast iron and 
should be welded or provided with Dresser couplings in place of screw 
couplings. Sewer lines should be laid with molded or poured flexible 
joints. 

ENTRANCES TO BOMBPROOF STRUCTURES 

Entrances to bombproof structures should be indirect, so that en¬ 
trance doors may not be exposed to the direct blast and splinter effects 
of nearby explosions. One arrangement which may be used is shown 
on the plan of a bombproof shelter. Fig. 159. This type of entrance 
will ordinarily not be practicable for power-plant doors through which 
large-size pieces of equipment must be moved. The latter should be 
arranged for convenient barricading and protection with sandbags in 
an emergency, and should be located on sides away from the open sea, 
so that they will not be exposed to shell fire. Entrance doors must 
be designed to resist blast pressures of nearby explosions and have 
some protection against reflected splinters. Edges and comers of 
concrete structures should be given a 6-inch radius or chamfer to 
reduce the pronounced shattering effect at these points from nearby 
explosions. 

TYPES OF BOMBPROOF STRUCTURES 

Views ar(‘ here shown of three typical bombproof structures de¬ 
signed by the Bureau of Yards and Docks. 

Bombj>roof Power Plant 

Figs. 155 and 157 show a layout, transverse section and perspec¬ 
tive of a typical bombproof power plant at an outlying Naval Station. 
Collective protectors are not considered practicable, and operating 


21 •• 


179 



I •< 
4 



SHOCK PROOF MOUNTING 

Fig. 164.—Shock proof machinery base and mounting. 



















































180 



Fig. 155.-Bombproof power plant, 
























































































































































































































181 




420504° 41 


13 


















































































































































































182 


ptTsoiinol are provided with gas masks for protection against ^as 
attacks. 


Bombproof Communication and Command Center 

Figs. 156 and 158 show a layout, elevation, transverse section, 
and perspective of a typical bombproof communication and command 
center for a small outlying station. This structure has complete gas 
protection, is air-conditioned and entirely independent of outside 
sources of power and water. 

Bombproof Personnel Shelter 

Figs. 159 and 160 show a layout, elevation, transverse section, 
and perspective of a typical bombproof personnel shelter with a ca¬ 
pacity of 200 seated or 400 standing. This shelter likewise has com¬ 
plete gas protection and air-conditioning and is equipped to serve as 
an emergency first-aid dispensary. It will usually be located adjacent 
to the station hospital. In all these types of bombproof structures 
the space between roof slabs is usable for shops, storage, or other pur¬ 
poses not requiring complete protection. It will be noted that bomb¬ 
proof structures need not be architecturally unattractive. 

Underground Bombproof Structures 

A word is in order concerning underground structure's, as they are 
frequently proposed as protection against bombing. Structures may 
be placed underground for concealment or to make use of the over- 
lying earth cover, if of appreciable thickness, as a medium of protec¬ 
tion against high explosive bombs. It might be advantageous to 
place structures underground, even though not much earth cover were 
provided, for concealment and as a substitute for complete protection 
against direct hits, if the structures were in an area which would not 
be suspected of containing military objectives and, therefore, not sub¬ 
ject to bombing. If they are, however, in an area which will in any 
case be bombed, reliance can not be placed on the concealment fea¬ 
ture' alone for protection, since the structures are as likely to be hit 
by accident as by intent, and consequently, if the facilities in the 
structure are important, adequate protective thiclmesses of earth and 
concrete must be provided against direct bomb hits. Earth, even 
when compact, is not an efficient protective material, being approxi¬ 
mately one-eighth as effective as a high-grade reinforced concrete. 
Tunneling or deep excavation and the concrete roof construction 
required to carry the heavy earth covering overhead make the cost of 
underground bombproof structures as high or higher than above 
ground structures. The underground ramps or entrance stairs are 
particularly costly. Underground structures are, as a rule, damp and 


183 



Fig. 157.—Bombproof t)o\vor plant 



Fig. 1o8. —Bombproof communication and command center. 











184 



LlI 

CL 

O 


CO 



ELEVATION 



Fig. 159.—Bombproof personnel shelter. 




















































































































































































































185 




Fig. 160.—Bombproof porsonnol shelter. 


subject to flooding if they are below sea or ground water level and are 
ruptured by bomb hits. This would be the case at most Naval Sta¬ 
tions, since the ground level near the waterfront is only about 10 to 
15 feet above sea level. Very deep underground structures afford 
good protection for vital services and communication connections, but 
equivalent or perhaps superior protection can be provided more con¬ 
veniently and at less cost in the case of above ground bombproof 
structures by multiple connections. The security of the connections 
is in any case no better than the degree of protection afforded every¬ 
where along the line. The conclusion is reached that underground 
bombproof structures at Naval Stations would be costly, inefficient 
from a protection standpoint, uncomfortable and possibly dangerous. 
Camouflage and concealment from above and from the sea is facili¬ 
tated by construction of bombproof structures against a hillside, and 
at a few stations where the topography was favorable, this arrange¬ 
ment has been followed. Camouflagf' is in any case in order, to con¬ 
ceal the character and importance of the bombproof structures. 

jMethod of Constructing Bombproof Structures 

The method of constructing bombproof structures follows usual 
heavy construction practice. Forms and shoring need to be substan¬ 
tial and well braced to support the great weight of concrete during 


186 


plaoino;. A donso well-gradod mix should bo usod, and tho concrete 
should be spaded and vibrated to insure imb(‘dment of th(‘ reinforcing, 
maximum density and strength of concrete and freedom from honey¬ 
comb. Because of the thickness of walls and roofing slabs, the use 
of the vacuum process of placing the concrete, is advisable, not only 
to obtain a more dense concrete, but also to remove the considerable 
quantities of contained water which, if not removed, may be ex¬ 
pected to keep the inside surfaces wet for a year or more. 

Degree of Bombproof Protection Provided 

While a structure desigiu'd in accordance with these recommenda¬ 
tions may sustain a direct hit of a bomb of larger size or striking at 
higher velocity than was assumed for purposes of design, and may, 
therefore, sustain serious damage, it should be pointed out that pro¬ 
tection has been provided against the largest and most destructive 
type of bomb now extant, of which few are in use, or would be likely 
to be used in general bombing of Naval Stations, that a considerable 
degree of protection is afforded by the inaccuracy of bombing from 
high altitudes, and that the design has been based on a number of 
conditions most unfavorable to the structures, all of which are un¬ 
likely to occur simultaneously in maximum degree. Moreover, 
should it later appear from advances in size of aircraft and bombs or 
from actual bombing experience that this amount of protection may 
not be adequate, increased protection can be provided by pouring 
an additional thickness of high-strength heavily-re in forced concrete 
on top of the upper roof slab. With this means available for aug¬ 
menting the protection, it is considered that the degree of protection 
proposed for the bombproof structures herein described, is all that is 
warranted at the present time. 


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